The team consisting of Dario FF, Chimera and TwilightZoney have released the first act of the first stage of Sonic Unleashed, Windmill Isle, for the PC version of Sonic Generations. The purpose of this release is to test the mod on as many systems as possible. If it works on enough systems they’ll start to convert all of Unleashed’s stages. More details can be found in the following video and in the link that follows.
Here is the forum thread with details where it can be downloaded along with instructions on how to install it. Other credits go to JoeTE for ripping the songs, Korama for making CPKREDIR (which makes developing and using mods more efficient and easier) and MainMemory for making Generations Archive Editor (which does what the name implies). Like it was mentioned earlier, the intent for this release is to test it on as much hardware as possible. Meaning that you can help the development of the mod by posting the results of how it works on your set-up on the forums. Also post results if you mess around with it by doing stuff like combining it with other mods like Classic Sonic Adventures or some model swap you found on Youtube.