Methinks that this seat is more likely to fall (whether Boswell reaches the General Election or not) than almost all of the Democrat gains of 2006.
While that may be a little of an overstatement, I completely concur with your view of Boswell's vulnerability. His 2006 performance was dismal for a three-term incumbent running in the best year for Iowa Democrats in half a century.
If the GOP can recruit a strong candidate (paging Jeff Lamberti...), Boswell, who's grown increasingly ornery and distant from his district in recent years, could face a stiff reelection challenge.
Do you think that a more active Democrat would have an easier time holding IA-03 than Boswell?
Yes, I'd argue a Democrat who more closely paid attends the needs of this district would likely be less vulnerable to defeat. However, since this district is essentially a 50/50 swing district, no Democrat can take it for granted.
I would think that any Democrat would likely hold this seat in 2008 if Democrats carry Iowa in the Presidential race.