As ftp masters came back from Debconf, rubygems package is now availablein the `experimental' distribution.
You need to add lines that follows to your apt-line.
deb http://ftp.debian.org/debian/ experimental main deb-src http://ftp.debian.org/debian/ experimental main
Then, install rubygems, which also installs libgems-ruby1.8
$ sudo aptitude install rubygems
The package source is now managed under pkg-ruby-extras.