The GPL is meant to give you freedom and stop you from taking away from other people the freedoms that you have received. You have to pass along those freedoms.
So if you are building an intranet site, you have the freedom to use OpenACS as much as you want. That's not a violation of the GPL. You also can modify it as much as you want, as long as you keep the copyright, GPL and credit notes there. You don't have to tell anyone that you modified it nor publish your modifications, as long as you are not distributing the software or trying to sell it to somebody else in a way that they can't access the changes (hard to do with OpenACS).
If you intend to sell that software to other people with your modifications, then you have to publish your changes. Otherwise you don't. But if you find bugs or things that you think could be improved, it would be very nice if you shared them.
