Basecamp is what my company uses for project / task management, and I use RTM for personal tasks. I could not live w/out RTM and my company could not live without Bascamp. Has anyone put together a solution for this (even if it's a total hack)?
Seems like you are not alone in your inquiry: http://www.rememberthemilk.com/forums/ideas/2917/
This is a good idea (from a user perspective) that seems not to have materialized yet. One would need to do an API integration of sorts between the two applications using a middleware app.
Likely the reason this has not been officially conducted by either party is that there is significant competitive featuresets and they would likely risk losing users one to the other.