With VMware, it is possible to VMotion a running VM across a WAN link. However, there are some "gotchas" you need to be aware of. For one thing, Cisco, VMware and NetApp all agree that is you want to VMotion a VM over the WAN, you need at least 622 Mbps. That is a lot of capacity, but isn't the main factor in this equation. The real "gotcha" comes in to play when you look at latency requirements. The same vendors state that round-trip latency for a VMotion over the WAN cannot exceed 5 milliseconds.
The speed of light in a combination of copper and fiber is roughly 120,000 miles per second. In 5 ms, light can travel about 600 miles. Since the 5 ms is round trip delay that means that the data centers can be at most 300 miles apart. That 300 mile figure assumes that the WAN link is a perfectly straight line between the source and destination ESX servers and that the data that is being transmitted does not spend any time at all in a queue in a router or other device. Both of those assumptions are unlikely to be the case.