Just to complement Jonathan's response.
ArsDigita is not developing ACS Tcl anymore. It's focus is completely on Java. But the OpenACS community will continue to develop and enhance it. The software lives on. OpenACS currently supports Oracle and PostgreSQL.
If you have software written for ACS (Classic), it will have to be modified to work with OpenACS. How much you will have to modify depends on how your ACS package was written, but most of it should work with OpenACS.
Regarding the GPL, you don't have to open source your modifications to OpenACS (or any GPL'd software). You only have to do so if you going to redistribute that software or sell it. In that case your modifications AND source code have to accompany your product.
If you incorporate (say) a proprietary product to OpenACS, that product does NOT have to become GPL'd, but the modifications you made to OpenACS so it works with that product have to be GPL'd IF you are going to redistribute those changes.
Let us know if you have any more questions. The community is very friendly and willing to help.
