In http://github.com/emerose/puppet/commit/71abfcb381e0ed141c270c6d9980e91161a4eba8 I've written a patch that will overload the 'source' parameter for the gem provider. If the source is a URL (with non-nil scheme part), the source will be passed to the gem --source option; otherwise, the source option will behave as before (ie, be interpreted as a path on the local disk where the gem to install can be found).
Note that the patch also adds an is_uri? method to the Puppet:Util::URIHelper module -- and that there's room for disagreement about whether it does the right thing (as there is about whether that method is useful outside the gem provider). In particular, URI.parse in the Ruby standard library interprets "/usr/bin/foo" as a perfectly valid URI that just happens to have a nil scheme component. A call to is_uri?("/usr/bin/foo") will return false, however. That seems right to me; if there's disagreement about that, though, I'd be happy to re-work this patch...
