I've recently been tracking some of Founders's limited releases (like the Kentucky Breakfast Stout, coming out in March I believe). It's easy enough to check brewery-by-brewery, but are there any services or websites that consolidate information about USA-distributed limited releases?
I'd say beermenus.com would be a good bet. My favorite local brewery updates their page every other day or so.
It's funny that you mention this though. I just had an email exchange with the brewmaster regarding this very thing. I asked him to set up a webcam system which will update the list of beer offerings in realtime (similar to what the Dogfish Head Ale Houses do at the bottom of the page). He liked the idea and said he's game for it, so hopefully we'll start seeing these type of things pop up more and more.
In the meantime, I think beermenus.com is your best bet (aside from calling the brewery probably). There's also a smartphone app called UnTapped that was pretty good at this as well, last time I checked (I'm happily a dumb phone user once again as part of a personal experiment of mine; so I haven't used it recently).