<update June 25> Microsoft added an interesting article about how Quick Storage Migration works
SCVMM 2008 R2 will offer a new feature called Quick Storage Migration. The virtual machine files can be moved to another location with some downtime. Also the live migration of VMware virtual machines is supported using VMware storage vMotion technology.
While VMware storage vMotion is only available in the Enterprise (available untill December 15, 2009) and Enterprise Plus editions, Quick Storage Migration is available in the far les expensive SCVMM license.
A comparions between both technologies
So here we are: Quick Migration, Live Migration, Network Migration, San Migration and Quick Storage Migration. A lot of migration types. An overview can be found in this article.