an example for the platform issue with rvm

rvm-platform-issue.sh

# switch to a rvm provided ruby version - creating a new gemset just to separate things
rvm --create 1.8.7@rjb-test

# then install rjb, this should work fine, installing rjb 1.3.1
gem install rjb

# now start ruby and require "rjb"
ruby -e 'require "rubygems"; require "rjb"'

# this should result in ruby terminating with the following message:
# [BUG] cross-thread violation on rb_gc()
# (null)
# 
# Abort trap

# cleanup
gem uninstall rjb

# now how it should be done
gem install rjb --platform ruby

# Have a look at http://rubygems.org/gems/rjb - there's a standard-1.3.1 and a
# universal-darwin-10-1.3.1. RVM just issues gem install and gem then decides to
# install universal-darwin-10. That version is obviously precompiled against
# Mac OS X's standard ruby and won't run with rvm's ruby (1.8.7 in the example
# above).
# 
# RVM should modify the environment somehow (now that could be the tricky part)
# so that gem doesn't fetch the gem suitable for the OS platform it runs on but for
# the ruby version it currently is set to use (ruby/jruby/...).

Glad I could help. I still do hope that this will be fixed in rvm. I'm afraid Wayne's moved on to other tasks, though.