I use Sugarsync and Google documents, and it occurred to me that these (as well as the various other 'cloud computing' systems might be fairly good third-party platforms to allow for file-sharing PBEM files.
The advantage here is that it could increase the convenience of PBEM to have files stored at a central repository (instead of e-mailing them back and forth). And, perhaps more importantly, implementation would probably be a question of refining the game engines' treatment of save game files -- i.e. allowing the user to specify where games are saved within their own computer's directory structure.