Not that I know of, but if you're a PC user, you can use a program called WinSCPto back up the sms.db file to your PC. The sms.db file is 5,120 in size and can be found in /var/msgs as well as in /var/root/Library/SMS. The guide to using WinSCP is here.
I was able to clear my messages on the iPhone and delete both sms.db files. I then copied the two backups from my PC on to the iPhone and the messages were back on the iPhone.
I realize that this doesn't really answer your original question, but in the future, this could be a way to back up those important messages in case, for instance, you have to restore your iPhone and wind up losing the messages stored on the iPhone.