I have the same problem with a few of my widgets, and also had the some of the header text in Mail displaying in fractions instead of letters.
I have thousands of fonts on my computer because I do lots of graphics work, and was able to disable the problem font that was causing the Mail issues.
I suspect there have been some system font changes in Leopard which may take some time to rectify. Widgets, many being 3rd party apps, may take until a new version to get fixed, or maybe Apple will issue an update to fix.
If you have a big font library, try clearing the font cache. I used a menu in Linotype FontExplorer, which is a font management application.
Thanks for the response. I do graphics work as well, although I don't have a font management program on here yet. (i do most of my work at work). I hope apple fixes this and I don't have to go meddling into my fonts to figure out which one is screwing up. I can't see any way to clear the font cache with font book either.