Last week saw the first evidence that Apple's iPhone OS 4.0 was already in testing per traffic reports on web site logs. This week, the OS seems to be undergoing further testing as one iPhone developer has since noted a single instance of iPhone OS 4.0 being used in their Pinch Media analytics.
iPhone OS 3.1.3 and 4.0 are apparently already in field testing according to a web site's access logs. The Boy Genius Report web site shows devices with iPhone OS 3.1.3 and 4.0 firmware connecting, alongside hardware at or before the present v3.1.2 revision. A v3.1.3 release could be imminent, as it would be a relatively minor update and just in time to support a flood of new iPhone and iPod touch owners.