Previously, I wrote that my Xperia X10i (AWS version, compatible with WIND) was frying SIM cards.
A little inaccurate, as it turns out. Full details are here, but basically the SIM is fine. It's the phone that's the problem.
After highway trips, or indeed any trip that involves a lot of cell site handoffs, the X10i seems to get confused about how to look for roaming partners. My current theory is that it gets stuck looking for a pre-programmed roaming partner that isn't there.
The long and short of it is: if your X10i doesn't want to work anymore, try cycling your SIM card through another GSM phone. Of any kind. This seems to 'unconfuse' the phone by resetting the information in the SIM and letting the phone work normally again.
Good news for anyone suffering with an X10i that periodically stops working. Try turning on airplane mode for long trips, and if needs be, keep a spare GSM phone around to recycle the SIM and get it working again.
I have no idea if the Android 2.1 upgrade fixes this or not. I've been too scared and busy to try it. I don't want to reinstall all my apps right now.