Recently screenshots of a prototype Amiga conversion of Sonic the Hedgehog were uncovered, thanks to Unseen64. The screenshots come from the Italian version of The Games Machine, dated September 1991, and show the title screen as well as a section of Green Hill Zone.
The screenshots are most likely mock-ups, say some members of Sonic Retro. However it has been confirmed that US Gold did buy the rights to the home computer versions of Sonic, meaning conversions for the Amiga, Atari ST, ZX Spectrum and Commodore 64 were to be produced. This was widely reported in magazines at the time, including Zzap!64 and C+VG.
One former US Gold developer has stated that the pictures are indeed screenshots from the PC version of the game, which would be used to develop the ports. The original article states that the magazine received the pictures as photos.
In my opinion, the screenshots do look as though they have been photographed from a monitor. Bending is visible due to the shape of the monitor, as are other features such as blurring that is common in screen photographs. However they could just be mockups created on a PC and then photographed.
It is unknown why the ports were cancelled, but one likely explanation is that the game was selling Sega a lot of Mega Drives so ports onto other systems would reduce sales. Sonic would certainly have been possible on home computers if it was done on the Master System, so technical limitations were probably not the reason for the cancellation. As of 2008, no prototypes have been uncovered and these are the only known pictures of the computer version of the game.