Ugh, I hate that craft store jean and lace design. *shiver* She's definitely reminiscent of FFX-2 Yuna and Aqua there though.
I'm going to assume the letter was from Rinoa and that all will be resolved, but we could just as easily have been trolled and it could never be mentioned again. Though, if she is from Radiant Garden it begs the question where she sent it from. What world did she escape to?
She may have been seperated from Leon when the heartless invaded Hollow Bastion, and never ended up in Traverse Town with him and the other FF characters.
As for what world she could have escaped to, that's still kind of a mystery to me.