As some folks commented, Google Voice seems to have stopped accepting outbound calls from Asterisk 1.8. There’s a bug report open with Asterisk on this issue, and several discussions in user forums.

I fired up FreeSWITCH again, without updating any code (still the source from mid-October, 1.0.head (git-dc40a77 2010-10-17 16-19-38 -0400)), and it works perfectly. Apparently, the FreeSWITCH guys are using a different method to make the Google Voice connection, or the Google folks are specifically filtering Asterisk or have made a change that affects Asterisk and to which FreeSWITCH is immune.