There is an announcement of WordPress 1.5.1 at WordPress Development blog. In short, there were over 170 bug fixes, 1 important security fix, and several speed improvements. Changelog is available in two formats: short version and long version.
I myself haven’t seen any WordPress bugs (or I can’t remember any), but I sure could use some speed-ups and query optimizations. This site is getting more and more popular and I can see the load on the database already. Hopefully it’ll get cut a bit with the new version. I’ll be upgrading shortly.