Can IOS 5 run on an iPhone 3GS 16GB?

Hello iPhone Gurus:

I have an iPhone 3GS 16GB model MB048LL that I picked up from my sister that I want to reset with the latest IOS for my son's use. Presently the phone has version 4.2.1 (8C148) installed. When I connect the phone to iTunes 10.6.1.7 on my Windows XP laptop, and press the "check for update" button in the phone summary page, I receive the pop-up "The version of the iPhone sofware (4.2.1) is the current version." It seems like there's no way to force an upgrade to IOS 5.1.1.

I'm a bit confused by this as the Apple website it indicates that 5.x is available for the 3GS. Also, when I connect my 4S and my other son's GGS 32GB phone, both shows IOS 5.1.1.

Any ideas? Is there an issue with the model and/or the amount of memory that precludes using IOS 5 on this 3GS?

Thanks,

Drew