I haven't seen anything in TCL 8.4 that breaks OpenACS. The real question is whether OpenACS will work with unmodified versions of the
AOLserver versions that include TCL 8.4. Roberto has just posted a
question about this to the AOLserver list (regarding internationalization specifically).
Check out Nathan Folman's AOLserver mailing list posting from a few days ago, which was also reposted on the OpenNSD thread.
https://openacs.org/bboard/q-and-a-fetch-msg.tcl?msg_id=0006Dk&topic_id=OpenACS&topic=
I've still not heard of anyone who's tested OpenACS with the AOLserver 4 betas in any sort of normal configuration. (i.e. non-Windows) I'm a bit surprised by this, given that people are already discussing the need for an OpenNSD fork. (Although I'm the first to admit that it could be necessary.)
