At this point, a new iPhone in September is pretty much a lock. The only question is how different it'll be from the iPhone 4.
The reliable Wall Street Journal is reporting today that it'll at least be thinner and lighter than its predecessor, according to component suppliers in Asia. It's also set to come with an 8MP camera and a wireless baseband chip by Qualcomm. If the chip is dual-band, it'll allow Apple to sell one version of the iPhone that would work on both AT&T and Verizon. Right now, you need to buy a phone specifically for one of the two carriers, and they won't work on the opposing networks.
Other than that, there's not a ton of new info on the iPhone 5. Will it feature a new casing, one less prone to cracking when dropped? Will the wraparound antenna that causes reception issues be fixed? We'll find out soon enough.