This is a fascinating insight that I haven't seen mentioned elsewhere. Our collective concept of what a "bubble" looks like is directly coupled with central bank manipulation, so to use the term in the same sense shouldn't actually apply to bitcoin.
That's not to say bitcoin isn't possibly overvalued at the moment.
the major historical manias had nothing to do with central banks.
also, while bitcoin may not be subject to 'this kind of manipulation', it is subject to entirely new kinds of manipulation. mt. gox could be making up trades that bolster the price. without information derived from a true marketplace with transparent, agreed-upon terms, we don't really have any idea what's going on. when i sold a large amount of bitcoins for dollars recently, i still held open the possibility that mt. gox wouldn't let me withdraw my money. fortunately, they did. but you'd just have to trust them to. we just have to exercise blind faith to hope that it is a market that roughly resembles the other (transparent, or at least regulated) markets whose charts we follow.