is it possible.. yes very.
is it plausible.. not really...
The concern here is that a pool operator will spend years building up his business and trust with the mining community, and then throw it all away for a one-time fraud. Keep in mind that someone would most definately catch on to what he did.. and once that became public knowledge, not only would 99% of his miners leave him immediately.. a good portion of those who left would DDOS and hack what's left of his pool into oblivion, just for the betrayal.
To put some numbers behind this.. a 51% pool would be mining 1800+ coins per day.. figureing in the current $160 exchange rate and a 5% fee.. that means he's bringing in $14,400 per day (minus infrastructure costs, lets call it an even $10k)
Would YOU throw away a $10,000 per day money machine to get in a double spend?
The only way this would be worth it would be for a million dollar double spend... Anyone who accepts a million dollar transaction without AT LEAST a dozen confirmations is an idiot .. and coordinating a dozen block fork would be very difficult even with 51%.
In short... I won't panic even if I do see a pool hitting 51% consistantly.
Sigg