I've been asked by my boss to find a solution that involves...
We create a VPN tunnel to a customers site. We then have a server running some virtualization software which is used to create as a clone of one of the servers at the customers site.
For example their Windows 2003 Domain Controller is fully replicated into a Virtual Server. If their server dies then we can move the Virtual Server onsite allowing them to continue to work whilst their main server is rebuilt.
There are many different ways to accomplish this goal. If you're looking to do some real time replication from the physical server...over to the VM thru the VPN tunnel, you might consider using replication agents from companies like NeverFail or XOsoft.
You could take snapshots of the physical server incrementally using PlateSpin's PowerConvert across the WAN. I'm sure that Symantec and Acronis can do this as well.
Many different options, it just boils down to the features you're looking for...and pricing.
I just finished a proof of concept for a customer doing this very thing. We used Platespin products, a wan acceleration product and their network. We were able to replicate every 6 minutes, bring up a test recovery, etc.
If you want to discuss this some more, let me know and I can give you more information.
Neville Data Solutions
Virtualization and Workload Portability Solutions