I have looked at two approaches so far, and find myself pretty close to the consensus (of -1.3% = -5.1% SAAR).
To put this in perspective, I am predicting that real GDP 2009 Q1 will be about $45 billion in less than 2008 Q4 in aggregate, or $150 per person. This is on top of the $230 per person lost in 2008 Q4, for a grand total of $610 lost per person in the last two quarters.
Productivity Approach (assumes that the Labor Productivity Residual declines about like it did Q3-Q4): Real GDP is 1.5% lower in 2009 Q1 than 2008 Q4 (SAAR = -6.0%).
Spending Approach (uses existing monthly data on consumption, construction, and government payrolls): Real GDP is 1.0% lower in 2009 Q1 than 2008 Q4 (SAAR = -4.1%).
Perhaps the biggest surprise is that 2009 Q1 real consumption is not much different than it was in 2008 Q4. When combined with the fact that government payrolls have not shrunk (private consumption and government purchases comprise the vast majority of total spending), it's difficult to predict that spending will fall much.