SMS2Mail allows you to organize, backup, and forward your SMS messages from your iPhone to any email account.
SMS2Mail will forward a log of your text messages via email in a set time frame and from one or multiple numbers to search from. There is keyword searching support as well.
It takes a bit to set up, you’ll have to manually input your SMTP settings for your email provider, I’d prefer it to attach the log and import it into a new messages in Mail. But once that’s set up you can send your log in a CSV file or in plain text.
[Source - BigBoss] Cydia