Good idea.
I think the first spot for plugins in bitcoin should be "send address resolver" services-- If I tell bitcoin "send 100 bitcoins to obama@whitehouse.gov" it could ask the plug-in resolvers (in some user-defined order) "do you have a bitcoin address for obama@whitehouse.gov" ?
If I tell bitcoin "send 100 bitcoins to eff.org" maybe a different resolver is used (DNSSec query to eff.org to get a bitcoin donation address, perhaps). Or "send 11 bitcoins to +01-1-805-253-2751" ...
There are lots of potential problems, of course, ranging from what if the resolver software service/author starts redirecting bitcoins to them to what if the user misspells the destination. That's why I think starting with a way of pluggin-in different solutions to try is the right way to start.