If @WalterGR above is correct, then changes to your site will only help when panda is rerun between one bot visit and the next (ie it's a batch process not a constantly evolved ranking).

If that's true then an awful lot of sites I have heard about may have been wasting time SEO'ing for nothing. (most obvious is serverfault)

Does anyone know if it really is an infrequent labelling run or an algoithm change and or it's run frequency?

Edit: reference to waltergr added correctly