From the poclbm source code, Switch.py line 219:

say_quiet('%s[%.03f MH/s (~%d MH/s)] [Rej: %d/%d (%.02f%%)]', (if_else(verbose, miner.id()+' ', '') , rate, round(estimated_rate), rejected_shares, total_shares, float(rejected_shares) * 100 / total_shares_estimator))

It's estimated_rate.

And how estimated_rate that found, you ask?
I tried looking at the source, but that didn't really clear things up.

self.estimated_rate = Decimal(new_accept) * (targetQ) / min(int(now - self.start_time), self.options.estimate) / 1000
self.estimated_rate = Decimal(self.estimated_rate) / 1000

I eventually found this discussion on bitcointalk. It states that the estimated rate is based on how many shares are submitted to the pool. I don't know what it's supposed to display if you're not using a pool.