Occassionaly Microsoft will allow a third party software vendor to produce a product that works and in this case NetIQ has one.
I have used their Domain Migration Suite and while not cheap, it does a fantastic job of effortlessly moving or joining domains together with automated scripts to bring the desktops machines and users under the new umbrella.
Documentation is outstanding, support is quite good unless you run into a quirky problem that is specific to your site, then it can take a little finess to get the 2nd tier to repond. When the do respond it is always with viable solutions and real answers though.
This from experience, I found the Microsoft way to be cumbersome and the documetation to be incomplete.