Though Apple's official line is that iOS 5 will run on every iPhone since the 3GS, as well as the iPod touch from the third generation on, and both iPads, it's good to see some evidence to back that up. The 3GS had a famously hard time adapting to life under iOS 4.
The video, embedded below, shows the developer trying out a beta version of iOS 5 on the 2-year-old phone. Since this is the very first release of the software to developers, however, we can't be absolutely certain which features are disabled on the iPhone 3GS.
There has been speculation that the 3GS's processor won't be able to handle the more advanced image-manipulation features, for example. Indeed, the video suggests there's no way to edit photos, but it may be that this feature won't be fully implemented until a later build of iOS 5.
Similarly, tabbed browsing is missing from the 3GS, but since we expect it to be useful only on the iPad's larger displays it would make sense for Apple to disable the feature on iPhones and iPods.
The video highlights a few software glitches where apps took some time to respond, but that's something most iPhone users are already used to seeing from time to time.
The iPhone 3GS has been spared this time but, much like previous models dropped off the iOS road map at version 4, we'd be surprised if it holds out much longer. After all, the first models were released three and four years ago, so they're positively prehistoric now. That said, even if the iPhone 3GS won't officially run all of iOS 5's features, you can bet there'll be a hack for that.
For the lowdown on iOS5, check out our iOS 5 complete guide, cross your fingers for panoramic photos and speech recognition and lament over what we didn't get. On the other hand, if you think all the mobile operating systems should get a look in, you'll love this smart phone OS comparison chart.