Before I go any further, I feel compelled to tell you I have zero contacts stored. This is not an issue of me not putting the area code in the contact's phone number.
That being said, I walked out of the store with my new 750, and the first thing I did was type my g/f a text message. I put in the 7 digits and it immediately came back "use the 10 digit number". Fine. I deleted the message and created a new one with her 10-digit number. She got it and replied, and I got the message a few seconds later. When I hit "reply", her previous message shows in the threaded view. There is no option to change the number, and it shows up in the title bar with no area code. I just don't understand this garbage.
So what I have to do is wait until the message comes back undeliverable, go into threaded view, hit the menu button and hit "resend". THEN I can change the number and add the area code.
Keep in mind, this happened with the phone as soon as I left the store with not a single contact stored. I was hoping after I upgraded it to WM6, the problem would go away... but it didn't.