I've been testing iOS 4 for the last week or so. So far, so good. Initially I thought my battery life went in the trash, but after I stopped playing with the phone so much, it soon went back to normal after I started using the phone like I regularly do, and stopped playing with it. I've been testing on a 3GS, so some features won't be available on non-supported hardware. Let's get to it. 

Some highlights:

Multi-tasking

If you have an app that takes advantage of the background processing APIs, you can now have tasks running in the background. As of this writing, only a handful of apps that I currently own can take advantage of this. Evernote offers this feature, ESPN Scorecenter, Dropbox (offers saved states), and a couple of others. In the coming days, I expect to see many apps updating, so be sure you check the app store frequently looking for them.

To switch tasks you simply double click on the home button and up pops a list of apps running. You can slide left to right to scroll through them. If you want to kill them, you simply hold down on an icon until they go into the jiggle mode and then press the '-' button on them.  

At this same screen, if you scroll all the way to the left, you will see your iPod controls (or whatever app is controlling the audio). This section also offers a software lock to keep the screen orientation locked into portrait mode.

Spotlight

 Spotlight search now offers to search for items on the web, including WikiPedia and Google. So, star typing 'Periodic Table' and you will see any items referring to 'Periodic Table', as well as options for 'Search Web' or 'Search WikiPedia'.

Folders

  Application management is so much easier now. Previously you could have 11 pages of apps, but no way to sort them. Instead of flipping through pages each time, I would just use Spotlight instead. However, I found myself forgetting what apps I had installed.

With the new addition of 'folders', you can now organize your apps anyway you like. To create a folder, simply touch an icon until it starts to jiggle, and then drag it onto another and it creates a folder with both apps. This folder will automatically be named with a generic category; Drop 'Twitter' onto 'Facebook' and it creates a 'Social' folder. You can easily change these names at any time, or drag apps out of the folder and back onto the home screen. I have gone from eleven pages of apps to three. Occasionally I find I have actually added a click here and there to launch, but overall, it's a wonderful improvement and I'm using my long lost apps.

Mail & Exchcange

Mail now offers a unified inbox. If you have more than one email account, you will immediately appreciate this. On the Mail screen it will show all of your inboxes, but is also has one option that says 'All Inboxes'. When you select this option, it shows mail from all of your inboxes sorted by the date/time of arrival.

On the Exchange front, the iPhone now is no longer limited to one single Microsoft Exchange account. This was always a problem to me, since I have a work email account that uses Exchange, but wanted to use the 'Exchange' configuration that Google's GMAIL offers. Before iOS4, I had to choose one or the other, but now they can co-exist quite well.

Spell Check

If you have an iPad, you've already gotten used to this wonderful add-on. No surprises on how it works, it's spell check after all. If you mistype a word it uses the red underlining and if you touch it, it offers suggestions that you can touch to replace it with. So far, it seems to work in all of the applications I have tested, even if they aren't 'iOS4' certified.

Cut, Copy, Paste, and REPLACE

This is also an iPad feature, and now instead of just the 'Cut, Copy, Paste' options, you now have an option for 'Replace'. Simply select a word and if it can find 'like' words, it will offer the option of replacement. For example, select 'that' and it will offer to replace it with 'than, that, or ghat'.

Bluetooth Keyboards

Although I haven't tested this feature, I have received reports that this does in fact work. I suspect we will see a case with slide out mini bluetooth keyboard now. That would be cool. 

Calculator

It received a new fancy icon.

Photos & Video

If you use iPhoto, you will be quite familiar with the 'Places' and 'Albums'. Now you can see all of the geotagged places that you have taken photos. It shows a google map with push pins and you can clicking on one of the pins shows an icon preview and how many photos are in this place. If you haven't chosen 'Faces', 'Events', or 'Places' to sync in iTunes, you will not see these options on your iPhone.

Maps

Google Maps has changed just a couple of cosmetic things. Now insteaed of crosshairs, you get the NorthEast facing icon in the bottom right hand of the app. Additionally, it matches what is in the titlebar, showing an app is using the GPS and is aware of your location.

 Summary

There are too many feature for me to hit on in a quick hit overview, but there is still plenty more new stuff to discover in iOS4. Check the settings menus to discover much more. Wallpapers, Search engines, SMS options, location services, etc. Tons of stuff to still discover. 

I've been using it for awhile now and seems very stable and I don't think you will have any problems with the upgrade. Remember, backup your phone before you do anything. These are computers and things CAN (and usually do) go wrong. Better to err on he side of caution. 

iOS 4 is released to the public today, so be sure to connect to iTunes with your iPhone/iPod and check for an update.  You will need to be running the latest iTunes 9.2, released last week.  Also, remember, depending on how much data is on your phone it could take awhile. My 16GB 3GS took approximately 2 hours to update and restore all of my apps. Be sure you have the time to do this, as your phone will be out of commission during the whole process. Good luck, and enjoy!.