WordPress MU

I've spent a lot of time trolling these forums looking for answers, and I've seen some stuff that's close but not quite it.

What I'm looking to do is to provide users the option to choose their main domain. i.e. on the signup form where they input their desired subdomain, i would like the main domain to be a dropdown box so they can choose subdomain.mydomain1.com or subdomain.mydomain2.com and so on.

I've seen methods for hacking around making other domains work after the fact, but is anybody offering this at initial signup? If so how? If not, anybody have any ideas?

Thanks in advance.

I have found this:
http://www.johnbreslin.com/wiki/index.php/WordPress_MU_Multiple_Domains_Modification

but it look too simple for me, and there is not written anything about how to set up multiple domains to work with only one WP MU installation.

I would be interested also in having 1 instalation of WP MU and to have options of ie. 2 domains for users to choose from when they create a new blog