Are there any arguments against running BizTalk on virtual servers?
BizTalk Server 2010 IS supported on a number of virtual platforms. 2009 are not supported on 2008R2 Hyper V though.
See the link here: http://support.microsoft.com/kb/842301
In the schema they explicitly lists Hyper-V, but at the bottom they list SVVP certified platforms are supported as well. That includes VMware esx from 3.5 and up.
What I normally see is that the BizTalk servers are ritualized, and the databases are kept on physical servers, to better control the performance of those... Probably 50% superstition to keep the db's on physical hw if you know what you are doing with virtualization, but it does make it a little easier to ensure your db's are getting all the performance they need.
The BizTalk servers ends up needing RAM and CPU, and that usually works well just being virtualized. But it all depends on if your people know what they are doing with vitualization. That is - not heavily overcommitting resources and such....
But what you REALLY want to read is this beast of a whitepaper, where Paolo Salvatori (BizTalk Ranger) and Ewan Fairweather had a LOT of fun virtualizing both SQL and BizTalk to see what happens to the performance. http://www.microsoft.com/downloads/en/details.aspx?FamilyID=0582bc67-0bef-4a0a-99cf-4408a111c4e3
[Spoiler] ... you don't lose very much performance when virtualizing....
One that may come to mind is the "support position" if actual support is needed from Microsoft. Will they support it as a virtual session/installation?