How Chafee voted didn't matter in 2006 since he had an R next to his name in a D year in an overwhelmingly D state. Besides Chafee wasn't as popular as Democrats on here are making him out to be. According to Rasmussen his approvals were only like 53%-45%
Such approvals are usually good news for incumbent Senator running for reelection.
Anyway, under good circumstances "R" next to Brown's name in November may play a factor too. We just don't know how would the situation look in November 2012 and assuming anything is at best premature.
By the way, unless many Democrats here, I'm not saying Brown is definitively going to lose, but he's not all-odds-safe either.
True to both of your points,
Given the state he's in and that it's early on Brown's clearly not safe. However, given the early indicators I do think the odds's are in his favour.
If there is more polling to back up what came out today, I don't know how you could say he is in a better position than Ben Nelson at the moment...