Long story short, subclipse requires subversion 1.5, but ubuntu 8.04 provides 1.4.x.
If you still want to use subclipse with ganymede in ubuntu, there are only two ways two achieve this – both of which i cannot recommend because they are hacks or bad compromises.

a.) You could add the the ubuntu package source of the upcoming ubuntu release (i.e. deb http://fr.archive.ubuntu.com/ubuntu intrepid main) and install subversion 1.5 and the package libsvn-java from it. This is a bad idea since it will also install a bunch of transitive dependencies not designed for ubuntu 8.04. It might in fact damage your installation.
b.) You could not use the java hl API but a software connector completely written in JAVA (SVNKit client). This will work at first sight, but will convert your svn repository into 1.5 format. As a consequence, you can no longer use the svn command line client, since it does not support this format.

Subversive as an alternative to subclipse

Facing the above problems, i looked for an alternative and tried subversive. It’s an official and free eclipse plugin. The developers intend to make it the default subversion support plugin for eclipse.
It installed without any problem and worked really well with the javaHL api shipped with ubuntu 8.04. Look and feel of the plugin do not differ much from subclipse – subversive even offers a couple of additional useful options.

Here is how i installed and configured subversive:

1. Follow the installation instructions on the subversive download site.
   Since you will use the native javaHL connector implementation, be sure to choose the javaHL implementation from the update site:
   
2. Install the java HL API on ubuntu: sudo apt-get install libsvn-javahl.
3. Add the library directory where javahl resides to your eclipse.ini (located in the root folder of your eclipse installation). Just add the following line into it:
   -Djava.library.path=/usr/lib/jni.
4. Restart eclipse
5. Verify your setup.
   Goto Window>Preferences>Team>SVN and click the “SVN Connector” tab.
   The native javaHL connector should be selected
   

I’m working with it since about a week, it’s fast, has not failed on my a single time and is easy to use.