After having a very lucky day for the pool that netted 30% more blocks than expected at their given hash rate BTC Guild pool operator Eleuthria commented on BitcoinTalk that "A lot of noise in the IRC, reddit, and forum yesterday related to "BTC Guild dangerously close to 51%" due to a large amount of luck yesterday.".
The concern is that if any one person or organisation were to gain 51% of the network hashing power they would be able to perform a 51% attack if they were so inclined.
The plan is to limit new users registering on the pool and failing that to disable the getwork protocol on the pool to limit users hash rate. Past that Eleuthria says there is very little he can do apart from encouraging users to use a different pool.
The full plan as laid down by Eleuthria is as follows;-
If Pool Speed is Over 40% of Network
BTC Guild will begin limiting the creation of new accounts. They will not be completely closed, but it will be throttled heavily. If speed continues to grow past 40%, the limit will be lowered steadily. I'm working on the system to implement these limits right now, and it will be ready by Monday April 8th, though I don't believe it would require activating that soon.
If Pool Speed is Over 45% of Network
BTC Guild will remove all getwork based pool servers within 24 hours. This is expected to reduce the pool by about 3.5 TH/s, or roughly 15% as of this post.
If Pool Speed is Over 45% of Network After Getwork is Removed
At this point there is very little I can do other than push notices on the website urging users to consider other pools, since the speed would be entirely based on old users expanding their hash rate faster than the rest of the network.Eleuthria - BTC Guild operator
While it is unlikely that BTC Guild would perform a 51% attack it is important for the bitcoin network security that no one actually reaches it.