If you do a lot of instances, you're probably familiar with the error message "Additional instances could not be launched. Please, try again later," and the accompanying dance of spamming the instance portal until it lets you in. For me it seems to happen most at the Azjol-Nerub portal, but that's probably just coincidence.
Anyway, although I've been seeing this error pretty much since WotLK launched, Wryxian just made a post today in the European forums explaining what it's all about. Basically, it's an attempt to fix instance performance; if too many instances are running at the same time on the instance server, lag and possible crashes result (and I'm sure we've all experienced that), so there's a limit imposed on how many instances can be up at any given time.