Leopard and Java 6 don't mix!
I didn't know this and used Java preferences to set Java 6.0 as my preferred version (I had earlier installed the developer preview, under Tiger)
The solution is to expunge all traces of Java 6, then reinstall Java.
Javablog has the answer:
Move or remove all of the following
/Library/Java /Applications/Utilities/Java /System/Library/Frameworks/JavaVM.framework /System/Library/Java /System/Library/CoreServices/Jar\ Launcher.app /System/Library/PrivateFrameworks/JavaApplicationLauncher.framework /System/Library/Frameworks/JavaEmbedding.framework /System/Library/CFMSupport/StubLibraries/JavaEmbeddingLib
and all your Java 6 installation receipts from /Library/Receipts/.
Then re-install Java from the Leopard DVD with the commands
open /Volumes/Mac\ OS\ X\ Install\ DVD/System/Installation/Packages/Java.pkg open /Volumes/Mac\ OS\ X\ Install\ DVD/System/Installation/Packages/JavaTools.pkg