my recommendation, have the client set their domain name to be a cname of their subdomain blog.
For example, on wpmu site MainDomain.com, user Ken get's the subdomain "Ken.MainDomain.com". Ken wants to use domain mapping to server up Ken.com. I would have him make a CNAME entry so that Ken.com points to Ken.MainDomain.com.
I've ran large farms of sites and servers and my experience is anytime you have the client setup by IP, migrating to a new server or datacenter is a pain because you have to get all of your clients to change their IPs at exactly the right time.
If instead you're using a cname, then you're completely in control of when you want to move and how it's done.
Plus, if in wpmu you want to have sites served by different servers, you're completely in control of this because you just move their subdomain IPs to whatever server you want to actually serve their site, and their CNAME naturally just follows along.
It's what I'm having all my clients do, but I have to tell them to ignore the A record instructions in the domain mapping settings. It would be better if I could edit those instructions to say whatever I want instead of making my users think using the A record is a good idea.