Block iPhones from receiving DHCP assigned IP’s

We have a subnet mask of 255.255.252.0 that
allows 1022 IP addresses of which we only dynamically assign 600.

This was plenty until this morning when all 600
were leased out.

Upon inspecting the leases – half of the
addresses were assigned to iPhones and iPods.

We could increase our subnet mask, but we would
rather block the iPhone devices from receiving IP’s all together.

We currently use 3 DHCP servers running Windows
Server 2000 and 2003. But we also have a Cisco
PIX which could do the DHCP if we wanted.

Any thoughts?