Duke University claims a single iPhone managed to knock out as many as 30 Cisco wireless access points for 10 to 15 minutes:
As it turns out, the iPhone does appear to be causing some network problems at Duke University, although it's the wireless LAN (rather than the West Coast network) that has been affected. The problematic traffic floods have occurred twice now, and in both cases a single iPhone was apparently enough to knock out as many as 30 Cisco wireless access points for 10 to 15 minutes. It's unclear what caused the phones to begin sending out thousands of Address Resolution Protocol (ARP) requests per second (yes,thousands per second), all of which are asking for the MAC address of a IP that isn't on Duke's network. One theory is that the loss of wireless connectivity or the change to a new access point causes the iPhone repeatedly try to reconnect to a router that it's seen before, but no one has come up with a definitive answer.
Despite the lack of a definite cause, the university has been adamant that the problem isn't related to the Cisco access points. But even if the access points are fine, the problem could be related to Duke's network topology, or the particular way that the iPhone is interacting with their network. For now, Duke is pointing the finger at Apple and claiming that a bug in the iPhone is to blame, but we probably won't know more until Apple responds to the trouble ticket and/or issues a fix.