It looks like everyone's working in Ruby space, which isn't a bad thing. I'd sure like to see some work going on with Valgrind and Ruby though. I picked it up a while ago, but without much success. Maybe it's time to try again.
I've also seen some buzz starting to grow about a DTrace provider for Ruby. And now that DTrace runs on OS X as well as OpenSolaris, there should be a lot of Ruby hackers starting to use it. I don't know enough about DTrace, but would it be good for isolating memory issues?