There is a physicist out there who has examined financial bubbles and seems to have a pretty well-regarded mathematical test for them. He's found that the majority of financial bubbles have a certain mathematical signature which betrays the underlying cooperative "herding" behavior among investors. Examining both the U.S. and U.K. real estate markets he finds evidence of a bubble forming in the U.K. but not the U.S..
The point of this is not to say the U.S. house prices are sustainable - I don't think they are - but only that by one measure a classic speculative bubble has not formed.
Maybe what we saw in the U.S. was a bubble in mortgage lending (refinancings and new loans).
peace,
boddi