Here is the paper by Romer and Bernstein with their estimates.
http://otrans.3cdn.net/45593e8ecbd339d074_l3m6bt1te.pdf
This was before the actual bill was passed so some changes I am sure but you get the general idea of what they are thinking. Krugman (Nobel laureate) thinks the numbers are approximately right. I think he went a little lower but not significantly.
Romer is an often referenced economist. I've read about 400 economics/business books and come across her name quite a bit. The most recent being "Recessions and Depressions" by Knoop which I highly recommend.