Basically DT a2sd just sets up sym links from the normal place the apps folder would be to a location on your ext2/3 partition on your SD card. From the perspective of Android it just thinks the folder is in the normal location it is supposed to be then (even though under the covers the underlying Linux OS is actually reading/writing the SD card). This applies to both the apps plus there data if they store it in the normal location (some apps write their data to the normal part of the SD card in which case the data would still go there).
So, if you switch ROMs and that ROM has A2SD support, it should detect the SD-Ext partition and re-setup this sym link for you when you install the ROM and you will then have all your apps back (plus any uninstallable apps the ROM includes should go there). If your ROM doesn't support A2SD out of the box, as long as the kernel supports ext2/3 (whichever you formatted your SD card with) you should be able to flash DT a2sd after you flash the ROM and then run the a2sd script as root to set up the link to put the apps on the SD-ext partition again (I think this will preserve the apps that are there, but honestly haven't done it myself so you should make a nandroid of the SD-ext partition before doing this if you want to make sure you won't have to re-install your apps).
Hope this helps some.