Colin,
We have a customer with 80 IAP-115's and 10 or so IAP-215's running 6.4.2.6-4.1.1.11. The AP's reboot quite often and the reason for reboot is Gateway unreachable.
Apparently the customer can ping the gateway from a port in VLAN with no issue.
I have read on Airheads that this problem could be something do with duplicate IP in the subnet, duplicate IP of the gateway or possibly a spoofed address of the Gateway.