If you think there are a lot of competitive Senate races this year just wait for 2012. We've found 9 of the 16 folks we've polled since October who are up for reelection next year with negative approval ratings.
It might be easier to start with the list of who's doing alright- the folks in positive territory in order of their net approval are Olympia Snowe (+15), Tom Carper (+13), Jeff Bingaman (+12), Herb Kohl (+10), Kay Bailey Hutchison (+3), and Jim Webb (+2)
Even within those folks who are doing comparatively well there's the potential for some competitiveness next time. Snowe's popular because Democrats like her and could have difficulty winning another Republican primary. Hutchison just showed she's capable of getting trounced in a primary and that could embolden a more conservative challenger to her, although it's doubtful such a candidate would be remotely on par with Rick Perry.
Most of the Senators with negative approval ratings aren't doing that badly given how unhappy voters are with politicians in general- it's quite possible that voters will like their 2012 challengers even less. Also most of them are Democrats and we're likely to see officials of that party have their numbers tick up as we get closer to the President's reelection campaign. Nevertheless none of these folks have numbers that will ward off strong challengers. Starting from the least bad approval numbers we've found to the worst this group includes Debbie Stabenow (-3), Bill Nelson (-3), Jon Kyl (-4), John Ensign (-6), Bob Casey (-7), Sherrod Brown (-7), Robert Menendez (-11), and Claire McCaskill (-13).
The one class of 2012 Senator we've found this year with truly atrocious approval numbers is Joe Lieberman, who posted a -42 spread at 25/67 when we polled on him back in January. Unless people really change their minds about Lieberman or he can find a way to get reelected with less than 40% of the vote he seems likely to be gone next time. Of course a big x-factor there is whether Republicans will run a serious candidate or let Lieberman be their defacto nominee again- but the Tea Party crowd seems unlikely to accept someone who voted for the health care bill in the end as its nominee.
Yes it's early- but in the summer of 2008 we identified Colorado, Florida, Ohio, North Carolina, and Missouri as states that were likely to have competitive 2010 Senate races based on the approval numbers of the incumbents there and it has played out that way- so the kind of numbers we see now do show where the opportunities are for close contests. Whether they actually end up being so depends on candidate recruitment, shifts in the political climate, etc.