The forum's system files have been upgraded from YaBB SE 1.5.1RC1 (RC = Release Candidate) to 1.5.1 Final (which was released on April 5th).
Well, no sooner did I install the upgrade to 1.5.1 Final than I noticed tonight that YaBB SE released a 1.5.2 upgrade yesterday. (Thankfully this one didn't require me to reinstall mods and took all of about a minute to install/upload.)
Most of the changes are internal and you most probably won't even be aware of them. However, a few of the things you will notice are that previously fatal errors have been replaced with pop-up alert windows to remind us of what we've forgotten to do. No longer will you lose the text of a post or IM because you've clicked Preview or Send Message but forgot to type in a subject or the username(s) of the person(s) you're sending an IM. And an alert will also come up now if you've forgotten to type in your current password before modifying your profile.