After installing Snow Leopard, all my menu bar icons disappeared. After some digging, I found out that version 1.3 of iStat Menus is not compatible with Snow Leopard (version 2.0, released shortly after Snow Leopard shipped, is compatible).
I had a problem, though, because the uninstaller is located within the iStat Menus System Preferences panel. Since the panel would crash every time I tried to open it in Snow Leopard, here's how I solved that problem. First, I downloaded the 1.3 installer from the web site, Control-clicked on the package and chose Show Package Contents from the contextual menu, and found the uninstaller tool in the Contents » Resources folder. As soon as I had this uninstalled, my menulets returned.
So make sure you upgrade to version 2.0 (assuming you're running 10.5) prior to upgrading to Snow Leopard. Alternatively, uninstall iStat Menus 1.3 before upgrading.
[robg adds: I modified this hint quite a bit, as it was submitted prior to the release of iStat Menus 2.0.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090831123221574