That is an enhancement of the latest rubygems. The 'gem' method does
not trigger auto-require as 'require_gem' used to. The
'acts_as_taggable' gem has autorequire set to 'taggable', which
activates all that stuff you expect. But since it is not called when
you use 'gem', you need to require 'taggable' explicitly after that.
Notice, that there is no need to specify the full path in require.
Hope it helps,
$ irb -rrubygems