I just completed the installation of the new Apple OS for my iPhone 3GS. I bought the phone a couple of months ago after Apple confirmed that the new OS would be compatible with the 3GS. The new OS -- called iOS 4 -- was an easy download from iTunes and the installation was a snap. The OS was made available this morning at 10 a.m. PDT so I had to wait until just after noon to download it here in the Midwest.
The new OS contains many, many upgrades, but the one I am the most interested in using is Folders. You can now combine your apps together into folders to ease the clutter on your desktop. I combined like-apps together such as Facebook, Twitter and Seesmic into a folder that Apple named "Social." I'm keeping the name, but you can rename the folder. I combined my Google Maps, Google, Gmail and YouTube apps together and named the folder "Google." I also have a "Reference" folder and a "Utilities" folder.
In addition to Folders, the new OS supports multi-tasking. I have not yet experimented with that one yet!