I recall people trying to install 2.0 for 3G on the original iPhone but I believe it gave an error.
You would be wrong. I vividly remember
Gizmodo posting the link to the 3G firmware a full day early (as soon as it appeared on Apple's servers), and upgrading both my phone, and my fiancée's phone. No errors - just really bad performance. I ended up having to restore both when the 1,1 specific firmware was released, because the performance was just that bad.
You can filter through the comments yourself - some users had successful installs, and some users error'd.
Show me someone updating their 3G with 3G S firmware and I might believe you. If people CAN install 3G S firmware on the 3G then do they get the extra features such as percentage indicator and extra apps?
Seeing as how
C-Net has posted the link with a big warning telling iPhone 3G users that it's not recommended for install, I would be inclined to believe they might've gotten it to install themselves (or it's a standard legal disclaimer).
Though, upon some further research, users here have and on hackintosh have gotten error messages.
If, in the circumstance that one would be able to install the 2,1 firmware on their device, they would not get the new features, obviously. The hardware's not there, but the OS is smart enough to know that (just as the "3G" option was hidden to those users who installed 1,2 firmware on their 1,1 device).
Again, I don't have the 3G; I was just trying to rationalize why the user might be having problems with 3.0. It was possible to install 1,2 on 1,1 hardware - I just figured with random reports of people having successful 3.0 installs (with the 2,1 version) that the same thing was going on here.