The main servers I would never consider virtualising in a high-end environment are SQL Servers - or any database server, for that matter. The demands they place on the Operating System could be sustained in a Virtual environment, but it is much better for performance-reasons if you consider retaining such servers as being installed physically - with no other roles on that server.
With regards to Exchange Server virtualisation, Exchange is very heavy on Storage capacity, but not so much on RAM and CPU power. You could probably virtualise a few Exchange Servers just fine; you must, however, remember to have the VHDs where the Exchange databases are stored on the correct RAID array formats.
As Brian has mentioned, Domain Controllers can also be a pain to virtualise, for the reason that they don't start until after the host Operating System has started. If you do virtualise your DCs, you would want to make sure you have at least one physical DC which isn't virtualised - and make it a GC and DNS Server - to counter-act this issue.
-Matt
