It has happened to me a few times, but only following the installation of 10.5.4 - it might be related, but it could also be because of other software I have installed since then. No discernible pattern as to what causes it or why. It also only happens in one account at a time (other accounts open at the same time don't start to have the problem just because my account has).
Btw, I am assuming (as it isn't very clear from your screenshot due to the white background) that the problem is the missing background to the text label?