Offering multiple Domains for users to choose from

Hi,

i wonder if there's already a way to have multiple domains within WPMU where users can choose from where their blog should be hosted on. Let's say you have domain1.com and domain2.com and domain3.com. When the user now creates a blog "mydog" he can choose whether to have mydog.domain1.com, mydog.domain2.com or mydog.domain3.com.

Is such a thing possible?

Thanks,
Frank