At work we have a legacy home grown CMS that I've been forced to maintain under protest for a little over 3 years. After yet another problem report, I decided to apply Plone as a possible solution.
20 Minutes later I had duplicated all of the functionality in the home grown CMS and by default added in search, 508, usability, performance, RSS, user groups, and so much more. Skinning it to look like a modernized version of the old CMS would be easy. The hard technical part would be migrating the data from the old CMS into the new Plone version, but with Generic Setup I don't expect it to be too hard.
Convincing the powers that be that this sort of conversion effort would be a good, cost effective thing will be the real challenge.
Anyway, this is the sort of thing that Plone 3.x feels made to do. I bet with a couple weeks of work this could be done. I'll keep my fingers crossed for now.