Seems like the accessories makers are making a head-start on iPhone 5C announcement. From Case Mate, iPhone 5C casing will be shipping from 20th September. If you have read on the Internet, the Apple event is on the 10th September. So, the 20th September date seems to be a tell-tale sign that Apple is shipping iPhone 5C.
BTW, when I tried to locate this casing on Case Mate, it could not be found. So, read this “news” with a pinch of salt. evleaks has also leaked out a Case Mate casing pointing towards that it is iPhone 5C.