redirecting legacy URLs to corresponding drupal path
I've just done a transition like you mentioned and the domain names changed. I didn't want the old urls hitting my drupal server so I setup a 2nd site just for redirects. At first I had a simple php redirect script that would map old urls to Drupal urls. The mapping came off the "migrate_map" tables that the migrate module generated.

Then someone on the team was concerned about the immediate SEO impact and preferred to keep the old site content available at the old domain name. So I copied the new Drupal site to a 2nd hosting account.

I added a "custom404" module which uses hook_init() to read the URI, detect a pattern, find the unique key in the uri, then lookup the Drupal nid using the migrate_map table and then use drupal_goto() to do a 301 to Drupal URI. 

On that Drupal site I show a "website has moved" message with a link to the same URI on the new domain. Pretty soon I'll change my script to just 301 redirect to the new domain directly.

I was initially toying with the idea of using a lower bootstrap level (to have faster redirects) but ran into issues with database connections. Got some great help here about that.