Hi, I'm having the same problem for a few weeks now, after x hours/days php
becomes unresponsive, I've already changed 2 different php-fastcgi spawning
scripts and tried php-fpm as well, switched between many version of the php 5.2
and 5.3 branches but the problem still remains so I don't think it's a php issue
even if that would be the most logical conclusion.

When php becomes unresponsive (502 bad gateway error) all the php-cgi/php-fpm
processes are still running and I can see them in top/ps aux, when I restart php
everything start to work again.

I don't know if it's releated, but while I was browsing this forum a few minutes
ago I got the 502 error with the same behavior it has when it happens on my
site, it lasted some minutes than someone took care of it. Also, take a look at
this discussion:
http://groups.google.com/group/highload-php-en/browse_thread/thread/a3809a50eba71a45
I think that this is a nginx bug, it would be nice if some developer could look
into this, I could provide error logs and such if needed, just let me know,
thanks!

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,40332,70942#msg-70942