I've never read the agreements, but any fool should know that the software that's going to run best is the software that came out while his/her device is current gen! Anyway, thanks for pointing this out.
The iPhone 3.1.3 update imposes limits on your OS upgrades. See below direct quote from iPhone Software License.rtf:
"Apple will provide you any iPhone OS software updates that it may release from time to time, up to and including the next major iPhone OS software release following the version of iPhone OS software that originally shipped from Apple on your iPhone, for free. For example, if your iPhone originally shipped with iPhone 2.x software, Apple would provide you with any iPhone OS software updates it might release up to and including the iPhone 3.x software release. Such updates and releases may not necessarily include all of the new software features that Apple releases for newer iPhone models."
User after user complained about Battery life cut after 3.1.3 update on iPhone 3G. See: http://discussions.apple.com/thread.jspa?threadID=2324303&start=0&tstart=0