There should be several ways of doing this. However, until SONY decides to release an update with JRE-support it seems (at least for now) that you need to install a second OS on your Playstation 3 to achieve this. PS3 is Java Enabled, but it seems this feature is connected with the Bluray drive instead of the userland. There are several options for PS3, but at least I am right now evaluating the Terra Soft Yellow Dog Linux v6.0 (YDL 6.0), which is one of the latest releases.
http://www.terrasoftsolutions.com/news/2008/2008-02-05.shtml
http://www.terrasoftsolutions.com/support/installation/
http://www.terrasoftsolutions.com/support/configuration/
It's available as a free download through the mirrors:
http://www.terrasoftsolutions.com/support/downloads/
This should enable you to run JAVA on PS3. A separate JRE may need to be installed (haven't tried yet) besides the 2nd OS.
I'll report more in the coming days.