So, at times a cleanup is in order. Here is what i’ve done to cleanup my merb setup:

$ sudo gem update --system
$ gem search --no-version merb | grep merb | xargs sudo gem uninstall -a # NOTE: removes all old version of merb
$ gem search --no-version dm | grep dm | xargs sudo gem uninstall -a # NOTE: removes all old version of data_mapper
$ gem search --no-version data_objects | grep data_objects | xargs sudo gem uninstall -a # NOTE: removes all old version of data_objects
$ sudo gem sources -c
$ sudo rm PATH_TO_GEMS/cache/merb* # PATH_TO_GEMS is the path to your rubygems install.  Mine is /usr/lib/ruby/gems/1.8
$ sudo rm PATH_TO_GEMS/cache/dm*
$ sudo gem install -r merb

This was taken from the meb installation instructions at: http://wiki.merbivore.com/howto/installation/gems