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.
The thinking is that players would rather wait for an instance that is going to run well than be able to immediately enter an instance that might be laggy or crashy. And I probably agree. However, I can't help but be frustrated with the way it's implemented; I'd like to see something like a queue for instances. As is, the best way to get into an instance as soon as possible is to continually run in and out of the portal, which is hardly fun.
Fortunately, they are working towards optimizing the servers so that this is no longer an issue; eventually, the instance limit should be lifted and performance should return to what we've always known it as. Until then, I can only advise that strafing is probably the fastest way to get in and out of a portal.