Factors that can exacerbate this issue or cause it in conjunction with the Mac OS X 10.6.3 update include Adobe Flash, Spotlight problems and outdated drivers, and problematic startup/login items.
Regardless, the most effective fix thus far has been a downgrade to Mac OS X 10.6.2.
Now we’ve received word that the forthcoming Mac OS X 10.6.4 includes new IOUSB files that resolve the freezes/stalls under most circumstances. We expect this update to be released later this month.