As previously described, my Xperia X10i sometimes refuses to reconnect to WIND Home after trips outside of the WIND zone. Seems the phone gets confused as to which network to try.
The 'trick' of recycling the SIM card through another phone - GSM or AWS - does work. I've managed to fix my card a few times that way. Today I stumbled on a better way that doesn't involve a second handset.
1. Go to launcher and start "SIM Toolkit".
2. My version has "English", "French", and "Roaming Setting". Select "Roaming".
3. Tap "Select Visited Network".
4. Reselect "Roaming Setting".
5. Tap "Automatic".
The Xperia should re-acquire WIND Home at this point.
Note your handset may be sluggish for a minute, as it "catches up" to the changes that happened while it was offline.
Hopefully this saves somebody some grief.