[1] Obviously some things would be more difficult with say, svn, but my point is, I use git to use github.
(edit, missing word, edit2, stupid asterisk formatting)
Having git be universal in the Ruby/Rails world has been a huge accellerator for maintaing open source projects. No longer are we subject to core teams losing interest, anyone can fork and become the new core team.
It's not like it wasn't possible to fork stuff in the past, it just required a little bit more effort... enough so that you didn't just get random copies of stuff all over the place.
I wonder if you get a projects bugs if you fork it?
An indicator for the original code would maybe be useful, but it would lead to confusion when, for example, the original maintainers fade away and another repo becomes the pseudo-canonical tree.
Furthermore, trying to implement fork queues, pull requests, and even proper contributor attribution atop Subversion would be a ridiculous amount of work for very little gain.
Regardless, if you really want to stick with Subversion, you might at least consider setting up a cron job that fetches to a local git-svn mirror and then pushes a clone to GitHub. You won't have to learn a whole new version control system, and your users will happily run circles around you with their ability to fork, branch, and merge before you can copy your source tree into a new tag.