To followup, the behavior described above is repeatable:

1. Use N800 on open wifi for a while.
2. Eventually link stops working (grr)
3. Attempts to reconnect, gets link-local IP (grr)
4. Repeat (3) endlessly to the same result
5. Manually attempt to connect to private wifi, which then gives me an unable to authenticate error as I haven't set it up properly.
6. An attempt to connect to the open wifi then works.

It would be interesting to replace (5) with an attempt to connect to a different open wifi network and see if that works, but there aren't any here.