As long as we are waiting for Mylyn commit message templates to appear in EGit (see bug #329515), I have hacked an AspectJ/Equinox weaving based bundle that injects the current commit template into the EGit dialog. This is certainly something that breaks the second the CommitDialog implementation changes, but it scratches the itch of us early adopters.
The code as well as instructions can be found on Github. A p2 repo is online too. Make sure to read the instruction as the installation requires a manual step after installation with p2.