Since the 9/16/09 Update, I have some users whose Acrobat process will hang if they try to open a PDF link.  It's consistent per machine - works fine every time on my machine, but other users will hang every single time.  The version of Acrobat/Reader is across the board, anywhere from 6 to 9.  I think it has to do with whether a pdf launches within Internet Explorer (rather than Acrobat) - that's what seems to cause the hang.  
 
In Evo, the file association is set to "I".

In our case, it's always when the user launches the pdf from the camera icon.  Sometimes I've seen 2 or 3 instances of Acrobat.exe in Task Manager, hogging up 99% cpu.
 
That same link that hangs Acrobat in Evo will launch fine in DBA.