Thinking about it a bit more, if Facebook messages are themselves free, you can set up a fake facebook account and run a desktop client which acts on special me

Thinking about it a bit more, if Facebook messages are themselves free, you can set up a fake facebook account and run a desktop client which acts on special messages, in effect achieving TCP over XMPP. This is probably no more than a week of work for one motivated developer. Or, given that wikipedia is free, you could use a private talk page to transmit data too. That could be even faster for individual files. Post a message to the talk page which a daemon polls frequently, then replaces the talk page with a base-64 encoding of the content you requested. Both of those require you to have a machine with unrestricted internet access, but I imagine unrestricted home internet access to be much cheaper than mobile internet.