Every now and then, variables just stop showing up in the Workspace pane. who and whos show that there are definitely variables in the workspace - I just can't see them in the Workspace pane. Only restarting MATLAB resolves this. Why is this happening?
I filed a bug report and never heard a word... I upgraded to 2011a, and haven't had this problem since. Because of this, because no one provided an answer and because I can no longer investigate others' further suggestions, I am accepting this as my answer.