A fast rise in U.S. home prices has some in the housing market murmuring the dreaded "B" word.

New numbers out Monday only add to that "bubble" hypothesis. The nation's top ten and top twenty market composites on the latest S&P/Case-Shiller Home Price Index recorded their highest annual growth rates since May 2006—up 8.6 percent and 9.3 percent respectively. "It is a solid rebound," said S&P's David Blitzer in an interview on CNBC. "I would not call it a bubble, but I'll admit a bubble is one thing you don't see when you're in it. You only see it after it occurs."