jamie - 6:39 am on Mar 5, 2013 (gmt 0)
I am just doing the exact same thing. The problem is because everyone's needs are different there is no real copy and paste migrate tutorial or code snippet (like there is for almost everything else code wise).
However once you have installed drupal and the migrate module, install the migrate_examples module which has four or five real world examples of data migration. I based mine on the migrate beer example. It's difficult getting your head around the way drupal works, and took me several hours of coding to get the first data migrating (and I have quite a bit of php xp) but if you've rolled your own CMS, you'll get there.
I am a bit knee deep in the migration right now, but I'll try to check back regularly if you have any questions.
Ps. Interestingly enough it was ergophobe who recommended drupal for a large site ;)