Oh, so it was a mistake? Bummer. I wouldn't have minded seeing Generations on the PC. Some past Sonic games have seen PC ports.

I'm certain it will come out for the 3DS.

A 3DS Sonic game is still being made.  It was listed as a project during E3 2010 (however early in concept), and in a Spanish Nintendo magazine, Nintendo Acción, this game was brought back to the public light.  It reminds us that it's a separate product from Mario and Sonic @ the Olympic Games 2011.
http://nintendo3ds.davidturnbull.netdna-cdn.com/wp-content/uploads/2011/05/Sonic-Nintendo-Action.jpg
Sure, it looks like we're not getting Generations 3DS, but we're getting some sort of Sonic game that's currently in progress.  Plus, Generations could also show up late (it initially releases at the end of the year) on the next console as a launch title.  We know next to nothing about the 3DS game, but here's a good place to check:

http://sonic.wikia.com/wiki/Sonic_3DS_(working_title)

along with Shinobi 3DS

I wonder if the game got canned and NP doesn't know yet!