Brown leads Mandel 48-40. When PPP polled the race in August his advantage was 48-33. Since then Mandel has turned a 13 point deficit with independents into a 3 point lead, and he's upped his share of the Republican vote from 71% to 78%. Brown gets just 7% support from GOP voters and it's going to be very hard for him to replicate his double digit win from 2006 without more crossover votes than that.
There's reason to think the spread between Brown and Mandel will continue to tighten. 14% of Republicans are undecided, compared to only 9% of Democrats. The high level of indecision from GOP voters is probably a function of Mandel's relative anonymity- only 33% of voters are familiar enough with him to have formed an opinion. As he gets better known those Republican voters are likely to move off the fence and into his column, tightening the race further.
The news isn't all bad for Brown on this poll. His approval numbers are actually up to a 40/35 spread from 37/34 on the August poll. And his 48% standing against Mandel doesn't represent any loss of support- undecideds are just breaking toward the Republican which was somewhat to be expected since most of the undecideds are Republicans. Brown's pretty darn close to that magic 50% mark.
Brown's probably going to have a tougher race than he did in 2006, and its getting closer, but all things considered he's in decent shape.
Full results here