Well it didn't take long to hear from iPhone owners who tried the 3.1.3 firmware update to fix their existing battery problems. Some people are reporting the same problems, if not worse after updating to 3.1.3 from 3.1.2 or earlier iPhone OS versions.
Previously, many users updated to 3.1 software so they could take advantage of MMS functionality on AT&T and found that an unwelcome side effect was shortened battery life on the iPhone 3GS. (Learn how to turn on MMS without updating your 3.0 firmware here. Some have speculated this has something to do with the iPhone losing and acquiring the 3G signal repeatedly which drains the battery.