For various reasons I won’t go into, you can’t get Subversion 1.5 to build on Ubuntu 8.04.  You can however install it from this alternative repository…

First, edit your /etc/apt/sources.list and add:

deb http://ppa.launchpad.net/clazzes.org/ubuntu hardy main

Then update the repo and install:

sudo apt-get update
sudo apt-get install subversion

Now go back into the sources.list and remove (or comment out) that repository:

# deb http://ppa.launchpad.net/clazzes.org/ubuntu hardy main

All done!

Thanks to Seffyroff for ferreting out this info, gleaned from various locations.  This reportedly works with both 64-bit and 32-bit versions of Ubuntu.