No 51% style attack is an unmitigated disaster unless it is coupled with a method of infecting and modifying all known copies of the blockchain. Wake me up when that happens. It could halt the system for a time though which would be highly damaging.
Let's imagine.. If I had 51% of the mining power... why would I destroy a gold mine producing me 1800 BTC / day. That would be over 1M $ / day.
If I were stupid enought, after I had used that mining power to destroy BTC and those 1M daily incomes, the hardware would be useless. NO WAY.
I'd rather keep mining.
What would really suck would be some method of regenerating private keys (via something like a high percentage of fautly PRNG's coupled with backdoor'd crypto weaknesses and much processing power (quantum or otherwise)). Something along those lines would probably spell the end of Bitcoin-I since it would attack the existing value stores.