Don't quote me here, but I BELIEVE your text messages are saved to the iCloud if you have DOCUMENTS AND DATA enabled.
I did a fresh install with iOS 7, no restoring anything but iCloud settings, but when I was all said and done (never restored my local backup) my text up to the previous nights sync WERE there.
Just make sure that you've sync'd and gotten no new content before you install. You can always push a new sync before loading 7.
I've never had my text messages restore when setting up as new.
Do you use messages on a mac maybe and they are syncing back to your phone?