Here's the letter and signatures: PDF [1].
Yes of course they'll still vote to fund what they oppose. Yes of course they'll cave under the slightest pressure. Take it for what it is. It's a start.

Links:
[1] http://warisacrime.org/sites/afterdowningstreet.org/files/Letter Obama Afghanistan 12.5.2012.pdf
[2] http://warisacrime.org/congress