After using jvm a bit, I'll admit it has a few things of which I am jealous.
jconsole: attaches to a running process, shows you current GC stats
over time, and also current backtraces for running threads [updated
every few seconds].

http://redmine.ruby-lang.org/issues/show/977
attempts the backtraces aspect of that, for 1.8.7
Twould also be a nice feature for 1.9, if at all possible.
The GC stats are possible if you compile it with the right settings,
in 1.9 [though I've never used them], so that's there.

Ability to "dump" all the memory of the JVM, and thus look through the
references for memory leaks [via references to unuseful objects].

There is a patch [2] that allows for the traversal of all the objects
in memory.  Any chance a patch for that would be acceptable? [I'd be
happy to submit one].
Thanks much.
-=r
[2] http://timetobleed.com/plugging-ruby-memory-leaks-heapstack-dump-patches-to-help-take-out-the-trash/