Having >50% hashing power doesn't let someone change the rules; that requires >50% of all nodes to agree.
There are some truth to this, there was an incident early in development it was discovered that some hackers had 20 something billion Bitcoins in their wallet, the system back then didn't account for more than 21 Million coins in existence, so couldn't do anything.
The developers re-wrote the client and asked all miners to upgrade, the 20 billions coins the hackers had became obsolete.
Well we are avoiding my point... Perhaps this quote is the most relevant to my initial thought... If government cant control what the currency is, and the power is in the hand of people, they will never be able to trust that Bitcoin will always be what Bitcoin should always be. Gold will always be gold. Useless as it is, but gold. All I mean is that even if they would love the whole bitcoin idea they would be suspicious of a possible INMENSLY HUGE scam made by 51% of nodes... And given whats in stake, this could happen...
Unless a United Nations Hashing Power (UNHP), controled the 51% ;-)