If you need to access the sms.db file for your iPhone sms messages directly via browsing the file system for your iPhone 3G / 3Gs simply access it through any favorite program (such as iPhone Explorer) and browse to this location:
var/Mobile/Library/SMS
There you will find the files related to sms and text messages (including sms.db)
If you want to backup your current state of messages, simply backup all the files in the folder SMS and you can resume is later on manually.