I can't say that I know how it all works, but I think I understand for the most part that your ROM needs to have lines in the init.rc to mount the ext3 partition on the SD card and do a few other things. Here are some good links. Thanks for checking it out. From what I can see this is still the best option, but maybe others can chime in?
10 Step Guide to DT A2SD
How to setup a2sd or apps2sd
How to add support for A2SD to your ROM
Developers: Adding Darktremor Support to Your Existing ROM | Facebook
How to isntall if not baked in (end user)
Darktremor Installation Instructions | Facebook
Latest version that I can find?
[Script][A2SD] Darktremor 2.7.5.3 Beta 04 [N1] (01/29/2011) - xda-developers
Comparison of the different scripts/terminologies
[GUIDE] How to get more FREE SPACE for your apps - Android @ MoDaCo