The first mention was reported here and a “continuation” of sorts was recently posted by Ken Penders in response to someone asking for season 3 of SatAM on his board:
No Third Season But….
… there IS a Sonic Animation Project currently in development. A story has been worked out, but beyond that I really can’t talk about much at this point.
The only other thing I can say is that we intend to compete on the level of a Monsters Inc. That means whatever is developed has to be done with the general audience in mind beyond Sonic fandom in particular, so don’t be surprised if the overall plot is more in line with what you’d expect from the first film of a series, rather than something you’d expect from a third season or later entry in a film series.
Source: Ken’s Comic Book Stories & Art