What I have seen is that my miners report normal activity with no connection problems and yet one of the miners has no accepted shares reported on my account log on Slush's pool. The first occasion showed over six hours since the last share had been accepted; the second occasion showed over nine minutes without a share accepted. On both occasions, the time since the last share accepted matched the time since a block was found on Slush's pool but not necessarily the last block. It appears as if one of my miner's shares were no longer be credited after the pool's 'block found' event processing.
On both occasions, I stopped and restarted GUIMiner for the affected miner and share accepted activity on my pool account log resumed normally. The shares lost were not recovered.
Yup. I reported this exact phenomenon last week (among other issues) and the general consensus among the self appointed spokespeople for the pool is "Nothing to see here, stop complaining, working as designed". It was repeated issues, no word from Slush, and an almost hostile atmosphere from those who either don't have problems or can't be bothered to notice if they have problems that led me to move most of miners to other pools.
I will say this, having my miners spread out on different pools and participating in pools with much clearer reward systems makes my daily reward look much more reliable so far.