I have switched pool - very odd because a few minutes later I checked my Slush account page and it said last share 3 hours ago (was 0 minutes a few minutes earlier when hashing on Slush!)... I don't doubt Slush's honesty or reputation - but can you now trust the actual mining mechanism to be working properly? (...)
I've been observing such behaviour at the beginning of a new round. When a block is found by the pool, the "last share at" tells a time near the beginning of the round just finished. This changes when the first share for the new round arrives.
Could it be that a new block had just been found in those "few minutes"?