Today I couldn't log into the Phi and after much messing around it was discovered that the IP address assigned by Windows 7 did not match the one stated in the quick start guide. Workaround is to manually assign the address 192.168.1.99 to the Phi in the Network Connections preference panel.
This problem is nastier than first suspected.
Day 1: Bridge between Phi and Wi-fi just worked, as expected.
Day 2: Needed to manually set Phi's IP address (see above).
Day 3: Manually setting both Phi and Wifi addresses and bridging won't work at all.
It's as if Windows networking is fighting any attempt to connect from the Phi to the outside world. It keeps "learning" new ways to screw itself up completely. I'm at a loss today, it just doesn't work at all.
This has been working intermittently ever since. One USB dongle is definitely dead, the other sometimes gets an address from the wifi router's DHCP service and sometimes doesn't. When it fails the dongle is assigned an address from the 169.254.x.x range by the PC.
Another few days until the new dongles arrive, should have the definitive answer at that stage.