Someday I'll probably jump over. At this point though it's not worth the penalty of leaving a homogeneous environment, or paying the learning curve tax on a language that's still rapidly evolving. After 13 years with PHP, and with the MVC frameworks out there for it, I can make it do what I need to quick enough.
It would probably be easier to switch if I had independent projects that I could spin without having to put both PHP and Ruby into the same httpd.
Here's some neat looking stuff I haven't had time to play with:
http://kohanaphp.com/ New MVC framework
http://www.cherokee-project.com/ New high performance httpd
http://particletree.com/examples/pqp/ PHP profiler built into pages
http://www.firephp.org/ "Firebug for PHP"