I'm convinced the Kindle WiFi is only usable for
- communicating with the Amazon Kindle store
- receiving content you emailed to your kindle's address.
- (experimentally) browsing the Internet.
I suspect that 2 is a subset of 1 anyway.
I'm pretty sure we'd have heard if Amazon had implemented SMB or FTP or any other such protocol. I imagine they want to keep the device as simple, secure and foolproof as they can and other protocols might pose more problems in those areas than they'd want to provide support for.
Warning: The above is largely one person's opinion and/or speculation resulting from a few week's use of a Kindle 4.
Note: As far as I know, the Kindle Fire is an Android (forked) Tablet. So presumably it can do things that the "normal" e-ink Kindle's can't.