I think EXPE, YHOO, AMZN, and EBAY's high prices can be summed up simply - flight to "quality" by funds invested in technology and internet sectors.
If they've got money they have to put in the market, there are fewer and fewer "quality" options available.
As a class of stocks, I expect these to be some of the last to fall, kind of like MMM and PG in the Dow.