Just follow the standard precautions concerning tradeoffs that you would for virtualizing anything. I have implemented at several customers utilizing a completely virtual environment.
The only word of caution I would throw out there is that I think it is common industry acceptance that a highly utilized database should no be on a VM. So if your environment is 500 servers or less with just some routine compliance jobs and application deployment, then I would say you are fine. Anything above that, and you should closely monitor your situation and consider a physical server for the database.
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We have been on VM since our begining with no issues to speak of. We have a SQL2005 DB on one VM and our app/reports server on another. We have had no problems to speak of.