first: try and avoid user.domain.tld as a web address.
why? problems when looking at mail. never use cname as mx. the best solution seems to be a mail address of email@example.com and web address of www.user.domain.tld.
web server with same name as a mail server = trouble.
this involves dns comprising:
* IN MX 10 mail.mailserver.net.
IN CNAME your.sharedwebserver.net.
its all got to do with cnames and mx records and so on, which i don't understand (but comply with). unless you know exactly what you are doing, do what i say!
you are right - you can do some really cool things with this - it starts with dns, but your web and mail server have an equally major part to play.
sorry to play the advocate but you will do far better to switch to apache if at all possible. check out "Mass Virtual Hosting" developed at least in part by demon.co.uk (one of our competitiors but respect for this!). this, in conjunction with unix accounts will produce very automated user accounts. see http://www.apache.org.
you want to be able to change the user specified domain. in this case, his involves changing the username. but, how often do you really need to change a website address?
surely you can simply change it by adding a new website and deleting the old when needed. this will probably happen maybe only once every 6 months max?
do you mean being able to have whatever.something and change whatever whenever you like?
try mass virtual hosting again:
*.something IN CNAME server.name.
*.* IN CNAME server.name.
then deal with tihs using mass virtual hosting.
haven't tried it tho...