Block? I dunno. Burn out, yes, sometimes.
I paint for a living, I'm constantly milking my brain for creativity, and sometimes--usually following the frantic convention season, when I've had to whip off fifty paintings in two months or something--I get purely burnt out on the act of Making Up Interesting Stuff. I'll do a dozen sketches, start ten paintings, and won't finish any of them. The motivation is just not there.
It's not block, per se--I have no problem getting ideas, I have ideas like a swamp has mosquitos, and I'm about as fond of most of them--but I run out of the passion to turn the ideas into something more. Every idea I look at and go "Bleh." But they're not bad ideas. Most of them would be fine if I wasn't mentally exhausted. Even very stupid ideas can make great paintings, but you gotta have the passion and the stubborness to get 'em there.
Generally I wind up waiting for THE idea, the idea that I'll really care about, which is akin to sitting in the swamp and waiting to be bitten by one particular mosquito. This is a stupid thing to do, and once I realize I'm doing it, I take a day or two off and go to the zoo or do some gardening, or something, and then I go back to throwing crap against a mental wall and seeing what sticks.
Thing is, I sort of suspect that my brain is not so much tired as hungry. It needs fuel. Creativity feels effortless sometimes, so we get this idea that it IS effortless. It isn't. Creativity doesn't come out of nowhere. You have to feed it. Read a lot. Do things. Feed your brain. Give it raw materials to work with.