Perhaps because the title doesn’t mention Python (though the tags do), Google brought me here when asking the same question but hoping for a bash answer. That seems to be:
$ dpkg --compare-versions 11a lt 100a && echo true
true
$ dpkg --compare-versions 11a gt 100a && echo true
$
To install a version of rubygems that’s at least as new as the version from lenny-backports in a way that gives no errors on lenny and squeeze installations:
sudo apt-get install rubygems &&
VERSION=`dpkg-query --show --showformat '${Version}' rubygems` &&
dpkg --compare-versions $VERSION lt 1.3.4-1~bpo50+1 &&
sudo apt-get install -t lenny-backports rubygems
Perhaps I should have asked how to do that in a separate question, in the hope of getting a less clunky answer.