Rumors are starting to circulate about the next revision of the iPhone; everything from front facing cameras for video chat to larger capacities have been rumored. While we don't know the hardware changes, evidence has surfaced that iPhone OS 4.0 is now being tested.
BGR has found evidence of the OS being tested in their server logs. The screenshot, posted below, clearly shows the latest version along with OS 3.1.3; this will most likely provide bug fixes for the current OS.
Even though very little is known at this point, it does show that Apple is testing its latest software and (most likely) hardware which would indicate an upcoming release. A jump to OS 4.0 signals a major upgrade for the iPhone platform which should bring many new features.
Apple, in the past, announced their newest phones in January at their Mac World event and released the product in late June. Apple is cyclical in nature with its product releases so don't be surprised if the pattern holds true.