IdeaMatrix might not be what I'm looking for, but ToDoMatrix looks really excellent for my needs: REXwireless Software
(Caution: that's a link to a video with voiceover.)
It looks more or less exactly like what I want, and it may not even require a data subscription. Thanks for the heads up on the student discount.
Edit: Actually, it does require data:
REXwireless BlackBerry software requires e-mail and internet data provisioned on the BlackBerry smartphone.