I've rebooted twice.  After the Messenger window comes up, the AT&T stay connected ad starts to load at the bottom of the window and then the Messenger window freezes.  Checking the Resource Monitor, it then proceeds to eat about 1MB a second from then on, suggesting a serious leak in the inline HTML advertisement.

In the time it took to write this post, Messenger has consumed an extra 50MB of memory, - it's now 'using' 460MB and rising.