Well, I may have found a solution. This has been bugging the p*ss out of me, too. I kept digging and digging and came up with the following fix, though I don't yet know exactly what causes it to begin with. (Note: I'm non-rooted on AT&T.)
- Go into Contacts and select a contact that is having the +1 format problem in his/her message headers.
- Tap the "messages" icon next to their cell phone number to start a new text message.
- Send a message ("test" or whatever else you want to say) to that person.
- After it sends, go back into your SMS messages list and select that person's message thread.
- The number at the top should now be formatted correctly (the way you have it in your Google account, if that's what you're using.)
- This continues to be displayed correctly even after restart/reboot.
I tinkered with this for about 45 minutes and it seems to work consistently for me. If you are using a different SMS app (e.g. Handcent, GO, etc.), you will need to choose which one to use when you start the message from your Contact list. I
*think* it works with those apps the same way, but I can't say for sure. When I tried it with Go SMS, it seems to get rid of the +1, but the number doesn't include the parentheses like they're displayed in my Google Contacts and in the stock messaging app.
Good luck, and let me know how it goes.