For several months we have had some problems with our Sales force being unable to communicate with our Exchnage server. Only Vista, Only Exchange. There would be intermittent communication, and then nothing, just like a wire was cut. After a full day with Dell tech support, they passed the case to MS. The MS tech immediately got some packet captures, looked them over, and found the problem.
We had an intermittent arp poisining, only effecting Vista, it was causing Vista to see the propper MAC address of the Exchange server, and then right after, was given the MAC of our PIX firewall.
Long story short, disableling arp proxy on the firewall fixed this issue. PIX has proxy arp on by default (WHY?!) and with the static translation in its tables from wan interface to internal server, it felt the need to speak out on arp requests. XP, Macs, and linux ignored it, but Vista BELIEVED its Hype, and was unable to tell a proxy reply from a authorative reply (or may have just been confused because it could hear the reply where the other OSes may not?).
Anyway, one less problem on my mind, one more PitA solved. Hope this helps someone else.