I recently upgraded Java to version 1.6 (build 1.6.0_13-b03-211) on my macbook pro running OSX 10.5.7 (leopard). It was a bit of a chore but it mostly sped things up and allowed me to run some of the newer apps that require Java 6.
I had a major casualty though when Aptana Studio stopped working. It would simply crash without even giving a decent error message and the logs were not helpful.I normally run the standalone version of Aptana Studio (1.2.7) which is built on eclipse 3.2. This seems to no longer run on OSX and Java 6 so I went in search of a fix. After lots of forum browsing, tweaking configurations, and reinstalling I ended up retiring version 1.2 and trying out version 1.3 (still in beta). It was hard to find the 1.3 downloads page so here is a link.
It installs quite easily and ran which was a major improvement. However, when I tried to install the features (plugins) I am used to, they all indicated that they were incompatible and would not install. This seemingly hopeless situation was actually easily fixed by updating Aptana Studio (Help -> Software Updates). Once that was done (definitely restart here) I installed the features (php, pydev, git) and plugins (epic) that I like and everything seems to work fine again.
Hopefully this will save someone a little pain.