I think the Presidential math is problematic for Scott Brown.
An analysis of his voting record in the Massachusetts State Senate suggests that he lies somewhere between Olympia Snowe (the most liberal Republican) and Ben Nelson (the most conservative Democrat). That might be fairly popular in the general election, but I can't see him ever making it through the Republican primaries.
The fact that he's from one of the most liberal states in the nation is also problematic for him. If he runs to the far right in order to make himself appealing to the Republican base nationally, he's just the most junior Senator and he pretty much guarantees that he loses his Senate seat in 2012 for a long shot at the Presidency. On the other hand, if he votes as a moderate Republican, he stands to become extremely powerful in the Senate as Senator #60 who can either make or break the filibuster
and he has a decent shot at reelection. After that, he's secured a role as a bipartisan deal maker, he could try to gravitate to the right and have a better shot in 2016 or later.