Ubuntu Backports. This team of volunteers oversees the procedure for publishing new versions in existing versions of Ubuntu. In the case of subversion 1.6, a bug has already been filed. If all you want is an installable package, a few people in suggest repos containing the backport.
If you want an official Ubuntu backport of SVN, well, it's a volunteer process. I've found two other related bug reports, and I'll clean them up a bit, but ServerFault readers can help by learning the backport requirements, learning the process and making sure all the ducks are in a row, and promoting it.
Best Answer
tortoiseSVN is very good.