The "credit crunch" that started on Main Street with the U.S. housing market has now spread to Wall Street. But the next question is how bad this liquidity squeeze will get: At it worst, it could terminate the buyout wave that's been a big part of the reason U.S. stock prices have achieved record highs this summer – sending U.S. shares into a tailspin. And that, in turn, would turn into a bear-market contagion that spreads to international market.