2*Difficulty is not much of a record. I remember when Deepbit (or maybe Slush) had a round that was 8*D.
BTC Guild, Slush, and Deepbit have all had rounds in excess of 10*D. Of course, that's 3 of the oldest pools, who have all at one point been at least 30% or more of the network hash rate, so they have had a ton of rounds in order to hit those numbers.
And if anyone's interested, here's how you calculate the probability of a maximum length round after
n rounds:
Pr(X <= x) = (1 - exp(-x))^n
For example, if a pool has solved ten thousand rounds and the maximum length round was 10*D, then the probability of this occurring is:
Pr(X <= 10) = (1 - exp(-10))^10000
= 0.6350766
What this means is that if that pool's history was rerun one hundred times, 63.5% of the reruns would have had a smaller maximum. Unless the result is greater than about 0.975, I would call the occurrence not unusual.
