Upon downloading the trial of CS5, it would not connect to my web-server. using passive (on or off), with new settings or anything.
I found un-checking the "Use FTP optimization" box sorted all my problems. It did say next to the box that this may help if you are having connectivity trouble, but just thought I would publicize this fact.
I have the full version (upgraded yesterday) and that function seems to be working fine. Can you provide more details about the setup? Windows/Mac? IIS/Apache? What error were you getting in the FTP log (Site->Advanced->FTP Log)?
I'm using OSX 10.6.3 -> Design Premium CS5.