Update: Solution to Vista Problem Accessing Office Documents from SharePoint Document Libraries That Are Published Using ISA Server 2006
Looks like I pulled the trigger too soon yesterday when I said that there was a solution to the Vista connecting to SharePoint through the ISA firewall problem. Turns out that had already tried this solution and it was a no go. In addition, Jason points out another useful post on this issue http://lumbgaps.blogspot.com/2007/11/office-2007-and-vista-persistent.html that includes another workaround, but not really a solution to this problem.
For the time being, there's not really a good solution to the problem, but the good news is that they're working on it. I'll be back with the solution when it comes in.
Thomas W Shinder, M.D., MCSE
Sr. Consultant / Technical Writer