I'm a little left in the lurch currently, because I need Subversion 1.5 client binaries in my Ubuntu 8.04 virtual machine (the one I use for training). This is because Eclipse check projects out in such with 1.5 compatibility (IDEA is kind enough to ask which version to use). In any case, since setting up my Tapestry Workspace involves a bit of command-line SVN, I found myself needing to upgrade SVN to 1.5.
With a little hunting around and some guesswork, I found the solution.
You need to add a new Software Sources location; this is a tool, "Software Sources" inside the Applications > System menu. Click the tab labeled "Third Party Tools" and add a new source. You'll be prompted to enter an "apt-get line":
deb http://archive.ubuntu.com/ubuntu/ intrepid main
From there, go into Synaptic Package Manager. Find package "subversion" and you'll see that there's a version of 1.5 now available, thanks to the new software source. You can right click and select "Upgrade", then apply the changes.