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Jaleel White, known for his acting as Urkel in the 90's sitcom "Family Matters," was considered by Sega to voice as Classic Sonic in Sonic Generations. White was chosen because he had previously voiced as Sonic in "The Adventures of Sonic The Hedgehog" cartoons. He declined the offer, leaving Sega with a silent Classic Sonic.

Source: <http://www.tssznews.com/2011/10/06/report-sega-considered-jaleel-white-to-voice-classic-sonic-in-generations/>

This needs to be confirmed somehow. Second hand information on a humorously toned article seems fishy, especially since it seems this is the only source.

Here is another source: <http://sonic.wikia.com/wiki/Jaleel_White>
Look under Trivia.