TheNextWeb is reporting that developers have shared with them evidence that Apple is testing the iPhone 6 and iOS 7. The evidence shared is weak at best and involves developers seeing new device identifiers and OS versions in their usage logs. It's important to note that these pieces of information can be faked, so take this with a grain of salt.
The biggest factor that lends credence to these logs is the IP associated with them. The IP address is one that comes from Apple's Cupertino campus and would be a harder piece of data to fake. Timing also lends some credibility to the data, with Apple usually releasing the first public version of a new iOS at the WWDC in June.
Lastly, rumors have said that Apple might update the iPhone as early as this June, so testing now is definitely right on schedule for that. Until a bit more evidence surfaces, it's a good idea to remain skeptical of iPhone 6 or iOS 7 claims and rumors.