How to provide a temporary URL for custom domain in WordPress multisite install?

I have a website with WordPress 3.0.4 installation, set up as multisite install. Some users register their blogs as something.mydomain.com and that works automatically. However, some users prefer to use their own domain names like something.com. This also works fine once they set up the CNAME record to point to my server. However, it takes 24-48 hours for that change to take effect. I’d like to be able to offer the user a temporary URL that would work out-of-the-box until the DNS changes are propagated, but I have not idea how to do it?