In The Hill today, Aaron Blake has a story about an important but little-discussed phenomenon that’s shaping the landscape for U.S. House campaigns in 2010: candidate calculations about the impact of redistricting after 2010:

As they enter a key decision-making phase of the 2010 election cycle, the chance that they will encounter a very different map in 2012 could serve as both a deterrent and motivation to go for it.
For some, 2010 might be their last best hope to win a given district before it is shored up by redistricting, while others might want to wait for the post-redistricting election, when the grass could be greener thanks to a friendly reapportionment process.

The go-for-broke temptation that Blake mentions refers to the strong tendency of legislatures to protect congressional incumbents, sometimes including those from the opposite party (often voters from the same party of a strong incumbent are “packed” into his or her district to make neighboring districts friendlier to the other party). Incubment-protection is a particularly strong impulse in those many situations where neither party completely controls redistricting.
But sometimes circumstances cut in the opposite direction:

Since redistricting often aims to shore up incumbents, it’s rare that it leads to better takeover opportunities. But that could be the case with members like Reps. Dennis Moore (D-Kan.), Peter King (R-N.Y.) or any number of vulnerable Illinois Republicans.
King and Rep. Mark Kirk (R-Ill.), who are weighing Senate runs, could actually be encouraged to run for statewide office because of the upcoming changes to their tenuous districts, which are likely to be handled by Democrats.

All in all, the wait-and-see tendency is naturally strong, since incumbents tend to settle into their districts between redistricting cycles. Moreover, among challengers, no one wants to risk an entire career to win a House seat and then see the district become unwinnable after just one term. Here’s Blake’s bottom line for 2010:

Overall, history shows that the number of quality challengers who emerge to run for Congress declines as redistricting maps become more entrenched, with the final election before redistricting – 2010, in this case – having the fewest quality challengers.
Vanderbilt political science professor Marc Hetherington, who has also studied redistricting’s effect on candidates, said the upcoming election cycle could be something of an exception, given the number of Republican seats that have flipped Democratic the last two elections.

It’s worth remembering that state legislatures are also redistricting themselves even as they draw congressional district lines. And that can have strange and interesting impllications, as Blake explains:

In perhaps the weirdest redistricting conundrum in the country, state Sen. Darrel Aubertine is a Democratic favorite to run in the upcoming special election for Army Secretary-designate Rep. John McHugh’s (R) upstate seat. But Aubertine could be risking his party’s control over redistricting by giving up his state Senate seat to run for Congress.
The Democratic majority in the Empire State’s upper chamber was always tenuous, but Monday’s coup by 30 Republicans and two Democrats put everything in focus. The 30 remaining Democrats will be fighting hard to regain their majority status and, provided Democrats retain the governor’s mansion, control congressional redistricting.
Aubertine’s district is one of the most difficult they hold. So, in effect, he could actually help his party win more congressional seats by staying in the state Senate.

As is often the case with redistricting cycles, candidate calculations going into 2010 may well involve many games of three-dimensional chess around the country. That complicates the national picture significantly.