Alternatively, instead of showing MHash for current block, save that value when the block ends and display it on the next block so it might read:
Hashrate for last round: 261 MHash
Actually it's not a bad idea. But in the real world, this will be still highly approximate for shorter rounds (don't forget that there are still people with <10Mhash/s, so they'll see "0 Mhash/s" for most of the rounds. And when users are OK with some hashrate delay, why not to use average for last 10 rounds, which is very exact?
Well, people are asking for actual hashrate very often and I feel that I should resolve this somehow. At this moment I'm thinking about new pool core implementing Stratum mining interface and also some extended features which I'm missing. As I see, providing actual hashrate should have some priority on checklist.