We're still hunting down hard info on this one, but according to users on AT&T's official support forums, Samsung Blackjacks manufactured between November 2006 and February 2007 are the subject of a quiet recall due to an antenna issue. Apparently certain internal components are wearing oddly over time and causing the antenna to loosen and disengage, causing dropped calls and poor signal strength. A service advisory has apparently gone out to warranty centers, so if your Blackjack is acting a little flaky, now's the time to get on the horn.
Update: Okay, we've gotten our hands on the service advisory. Like we said, it's all i607 Blackjacks manufactured between November 2006 and February 2007 -- the date is printed on a label under the battery in YY MM format.
[Thanks, Douglas and Curtis]