There's a serious bug in Tcl 8.4.* which is going to slow down final release. Any code using various FILE commands (this would include OpenACS) is at risk, though it is a concurrency problem so hasn't been showing up except under heavy load. However the concurrency error can cause the wrong directory to be used etc in a file command so is evil, in fact apparently it can cause memory corruption and an AOLserver crash.
Again ... this is a Tcl problem but AOLserver 4.0 requires Tcl 8.4.* so it's placing 4.0 on hold.
I imagine this bug my crop up as an item of discussion in today's AOLserver IRC chat session.