It's well-past time for The Office's websites to be upgraded and for the main public one, I'm considering migrating the existing .Net 1.1 ugliness up to ASP.Net MVC.
The only real doubt I have is over the fairly extensive work on SEO that has been carried out on the site.
What considerations should I bear in mind regarding SEO when moving from an old webforms site to MVC urls?
I would take a long hard look at routing. That's going to be the key to getting the SEO working as desired on the URL's. You may also want to have html helpers for formatting the rich url's.
You should consider your old links using either a) standard web forms with routing or b) making a large routing table that handles request for all of those old URLs that have already been cached / scraped by Google, Yahoo, Bing, etc. If, as you say, you already have some good SEO in place, you don't want all of those links to suddenly stop working.
If your question is only about URLs, then the answer is that redirecting the old URLs to new ones with a permanent redirect (301) should do the trick - search spiders should update the results to new links without any SEO hit.
There will be other factors to consider as well - e.g. changed HTML structure of pages (I hope you are not going to mimick the output of the old ASP.NET in your new web app :)).