As much as it pains me to say it, Sonic is more based on speed and not fighting. I would love to have sonic (and of course an alternate costume of shadow) in brawl, but i do not think he is brawl material. All of his game have been about running. in most stages there is barely any room to run. I love Sonic, but i do not think that Brawl is his place.
As much as it pains me to say it, Sonic has actually had a fighting game of his very own. It was even in a 'proper' fighting game style, like much like the classic Fighting Vipers and Virtua Fighter games, which are generally
MUCH slower than what Smash Brothers is used to, so I have a pretty good feeling that Sonic is more than able to fight in Melee... also, I think Sonic should be able to control how fast he walks and runs, don't you?
http://en.wikipedia.org/wiki/Sonic_the_Fighters
Also, I'd like to enlighten you on the fact that quite a lot of characters in Smash Brothers shouldn't really fit in Smash... Captain Falcon and Fox had previously only shown driving/piloting expertise before Super Smash Brothers, as a light example. Also, isn't Smash Brothers all about bringing characters that don't normally fit together into a special fighting tournament?
So yes, Sonic can actually fit into Brawl really well. The fact that he's already going head-to-head with Mario in the Olympic Games proves that he can blend in just fine, and also acts as evidence for him having a good chance of actually making it... hopefully these sources that have risen today will be correct and Nintendo announces Sonic on Wednesday!