Do you even know what a 51% attack is? Do you know what can be done using it? Do you know what would happen if a pool tried to do it?
Let's do a mind experiment. Assume BTC guild, right now, has 60% of all the hashing power. Now explain to me exactly what they would do in order to pull off a "51% attack", exactly what they would accomplish by doing it and most importantly exactly why they would do it.
A 51% Attack doesn't have to be intentional, does it? If any pool had 60% of the hashing power, an attack would be forthcoming. The pool doesn't have to try to do it, it will, which is why the pools cap themselves. Blocks other miners solve would be orphaned left and right... The longest chain always win the conflict, right? That isn't to say it was done intentionally or with malice. But whoever has the upper hand would benefit from the block rewards and transactions fees of those resolved 'conflicts.' On the other hand, if a pool wanted to orchestrate an attack... well, I guess that's another topic.
No, that's not how it works. There is no problem with a pool having more than 50% of the hash power as long as they don't abuse that power.