This is the (relatively) easy part. The login form requires a POST, but it will be fairly simple to whip up an auto-login using curl. We can trigger such an action from dbus, as you say, or possibly from an if-up script on the wireless interface.

Before I rush into doing that, though, there's a problem with when and whether to connect to a (potential) hotspot in the first place; if we have the phone connect and log in by default, then it will do that every time it encounters the given SSID. Which is dozens of times within a half hour bus journey, and for none of them could I have maintained a connection to the hotspot for long enough to use it.

It's probably too much to ask the phone to deduce whether I am on a bus or not, but this is why I am thinking that I don't want to just configure the SSID in the N900 and let it autoconnect. This favours making a more fine-grained assessment of whether we want to try to connect to an available AP or not. Once I've got that figured out (playing with wireless-tools at present) then it should be fairly easy to script up the web-based login for any given provider, but that should be a bolt-on to a means of deciding when and whether to use a hotspot. I'm working on this in the background, but would welcome any thoughts in the meantime...