The google talk folks have a dedicated department to setup and run the Jabber servers that handle the request. Furthermore this deparment spend a lot of time (presumably) to integrate jabber with their google talk application, at least if you look when they first moved into this area.
Assuming someone is willing to pay two developers full time to integrate Jabber with OpenACS and set it up accordingly, I'm pretty sure you can expect more excellent results than google talk has to offer within a month, due to the fact that the OpenACS part is already pretty much covered.
What you are experiencing at google talk though as a user is that you download a client and start using it with the google servers, as the client is preconfigured to use google. But this is not the issue for OpenACS and Jabber. If you have the combination up and running, then it is no big deal to download the client and login with your screenname given at the OACS site into the jabber server.
I hope this helps to differentiate the two problems and shows where OpenACS is still missing something.
