I don't yet own a Tesla (I ordered a week ago), but I don't see why you couldn't use the free WiFi offered by public libraries, coffee shops, etc. For that matter, you could set up your cell phone as a WiFi hot-spot and use it -- but then the downloads would count toward your cell phone's data limits, so you'd almost certainly want to shut off the hot-spot feature and/or tell the Tesla to forget that WiFi site once the download completed.
Also, and this is admittedly anecdotal, all of my updates have come later at night - well past the time I'd be sitting near a starbucks or other public wifi.Problem with that is, you cant currently "request" an update. the car will get it when it gets it. You could sit on free wifi for a while and not know when the car would want to pull an update. Until tesla allows us to push a button and have the car request an update, sitting on free wifi or a hotspot is not a real great solution. Like some have said, wifi at a service center may trigger it.