Yes, there’s a new application called ServersMan@iPhone that will do just that. Assuming your iPhone is on, and is running that particular app, then anyone on the Internet can upload and download files to their heart’s content.
Files can be transferred via a web interface or the WebDAV protocol. I’m still getting my head around why anyone would want to open their iPhone up to the world, but I guess the developers at least must like the idea.
It’s only available in the Japanese app store at present, with an English version arriving in March.
(Via Read Write Web)