The most recent Subclipse Eclipse plugin (v1.8.x) requires SVN/JavaHL 1.7.x; however, running

sudo apt-get install libsvn-java

doesn’t quite get you there in Ubuntu.

Fortunately Dominik Stadler has created a PPA you can add to your software sources.
Before you continue, uninstall your existing libsvn-java:

sudo apt-get purge libsvn-java

Then proceed to add the PPA and reinstall the package:

sudo add-apt-repository ppa:dominik-stadler/subversion-1.7
sudo apt-get update
sudo apt-get install libsvn-java

You may also need to update your eclipse.ini to include

-Djava.library.path=/usr/lib/jni

(or possibly /usr/lib/i386-linux-gnu/jni or /usr/lib/x86_64-linux-gnu/jni depending on your system)

You can now upgrade Subclipse to 1.8.x
.. or save yourself the hassle and move to git