Failure updating to RubyGems 1.5.0 on JRuby 1.5.6

Hi. I have successfully updated rubygems to version 1.5.0 on all my rubies except for JRuby.

$ rvm list
rvm rubies
ruby-1.9.2-p136 [ x86_64 ]
rbx-1.2.0-20101221 [ ]
ree-1.8.7-2010.02 [ x86_64 ]
ruby-1.8.7-p330 [ x86_64 ]
=> jruby-1.5.6 [ amd64-java ]
Here is the transcript:

$ ruby -v
jruby 1.5.6 (ruby 1.8.7 patchlevel 249) (2010-12-03 9cf97c3) (Java HotSpot(TM) 64-Bit Server VM 1.6.0_17) [amd64-java]
$ gem -v
1.3.6
$ gem update --system
Updating RubyGems
Updating rubygems-update
Successfully installed rubygems-update-1.5.0
Updating RubyGems to 1.5.0
Installing RubyGems 1.5.0
~/.rvm/gems/jruby-1.5.6/gems/rubygems-update-1.5.0/lib/rubygems/gem_runner.rb:84: undefined method `load_plugins' for Gem:Module (NoMethodError)
from ~/.rvm/gems/jruby-1.5.6/gems/rubygems-update-1.5.0/lib/rubygems/gem_runner.rb:31:in `require'
from ~/.rvm/rubies/jruby-1.5.6/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
from setup.rb:25
RubyGems system software updated
$ gem -v
1.3.6
I have also tried gem install rubygem-update; update_rubygems and ruby setup.rb variants, to the exactly same effect.

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$ gem list

• • • LOCAL GEMS ***

activesupport (2.3.10)
bouncy-castle-java (1.5.0145.2)
brown_shoes (0.0.0)
cartesian (0.6.0)
facets (2.9.0)
hoe (2.8.0)
jruby-launcher (1.0.5, 1.0.3)
jruby-openssl (0.7.3, 0.7.2)
newgem (1.5.3)
rake (0.8.7)
RedCloth (4.2.3)
rubigen (1.5.5)
rubygems-update (1.5.0)
syntax (1.0.0)

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First reported on RubyGems and filed here by suggestion of Charles Nutter (http://help.rubygems.org/discussions/problems/479-failure-updating-to-rubygems-150-on-jruby-156).