i found lots of iPhone users have the same trouble. that is they can't backup the SMS to anywhere when the SMS box is full, then they have to delete all of them to prevent iPhone rejecting new SMS receiving. so here i introduce a useful tool, hope that it can help you

Tansee iPhone Transfer SMS
With Tansee iPhone Transfer SMS, you can transfer the iPhone SMS to computer as a txt file or a ANTS file before the event above happens. Before the transfer, you can choose to transfer from all contacts or a single contact, further more, you can choose transfer the SMS to a TXT file or a ANTS file. In ANTS file condition, you can protect the file with a password to keep your privacy. From now on ANTS file support edit feature just the same operation process as the SMS edit on iPhone. If you want print iPhone SMS, you can choose MHT file format, then open the file by Internet Explorer, print it just like shown on iPhone!

Tansee iPhone Transfer SMS can provide you:
* Backup SMS in iPhone to computer before iPhone reject receiving new SMS.
* View and Manage old iPhone SMS in your computer.
* View SMS in Text file format (.txt file), ANTS file format (.ants file), MHT file format (.mht file)on PC.
* Password protection support (ANTS file only).
* Edit SMS (ANTS file only)
* Print SMS (MHT file print by IE).

for more information http://www.converter123.com/iPhone-Software/Tansee-iPhone-Transfer-SMS.html
derect download link http://www.converter123.com/iPhone-Software/tanseeiphonesms.zip