According to Dave Leary and Steve White, MicroFrontier has confirmed a conflict between ColorIt 3.2.4 (their latest version) and UMAX scanners (specifically, the VistaScan plug-in which the scanners require). The only work-around for now is to downgrade back to version 3.2.3 (although this has the bug that limits you to 72 dpi). MicroFrontier is working on another update to address the problem.
Update: Fred Higgins had a similar prolem with ColorIt! and the Apple Color One Scanner 600/27. Downgrading to the previous version of Color it! fixed the problem.