One of the biggest threats facing investors in 2011

Suddenly, everyone's a bull on the US. The Dow has climbed back to where it was before Lehman Brothers collapsed in September 2008. And it seems that investors' attitudes have flipped to match. Nomura's 2011 client poll shows that 26% of the firm's customers now see America as the country "most likely to see a growth surprise on the upside" next year. That's almost double the percentage for anywhere else. Even arch-bear Professor Nouriel Roubini has just conceded that the US economy could grow by 3% in 2011. Yet a real US 'nasty' could be set to bite markets in 2011…