I've had this problem for quite a while. However it seems to trigger when there is too many things/block/something in the world you open. However when you open a new map there are no problems. (Though the nyan seed did freeze after a while.
Happens in prerelease 1.4.7 vanilla SMP; no texture packs or anything fancy.
Fully zoomed out map cloned two times; one map placed on item frame on wall; the other two copies distributed to two players who both kept them in their inventory hotbar
It shows the green arrow where the item frame is but the players don't show as arrows. I think it still lets you reveal unexplored locations if you are in the vicinity
it's rejecting the screenshots I took with F3, what now?