While the holy grail for grabbing files on the iPhone would probably still be Apple-made iDisk support (as rumored), there are other options out there. Microsoft's Live Mesh is supposedly going to be the very definition of cross-platform once they mange to get all of their clients done -- but file access for the iPhone is pretty much limited to the browser right now.
Add one more "Securely sync your files to a 3rd party server and then get them on your iPhone" option to the mix: Sugarsync. They have clients for Mac, PC, Windows Mobile (though that one isn't great), Symbian, and now: a Sugarsync client for the iPhone (App store link).
Although Sugarsync would like to say that their iPhone client supports pushing files to the iPhone, that's not quite the case. Your files are pushed from sync'ed folders to Sugarsync's cloud, waiting for you when you want to access them from your iPhone. Good enough, we say, since local file system access on the iPhone is still not really available.
Basically the only limitation here is the iPhone: it can view but not edit and it can only view a relatively small set of files, but that whole local file system bit. One nice bit: you can share any file on your Sugarsync cloud.
The service is $25 a year for 10 gigs, so it might be a viable option for those of you rocking a few computers and an iPhone yet hate MobileMe. For those of us who are shelling out for MobileMe already, it would be nice to think iPhone iDisk access won't be too long in coming.