Here is an excerpt from a good doc on DHCP for IOS AP's;
Setting up the DHCP Server
"By default, access points are configured to receive IP settings from a DHCP server on your network. You can also configure an access point to act as a DHCP server to assign IP settings to devices on both your wired and wireless LANs.
The 1100 series access point becomes a mini-DHCP server by default when it is configured with factory default settings and it cannot receive IP settings from a DHCP server. As a mini-DHCP server, the 1100 series access point provides up to 20 IP addresses between 10.0.0.11 and 10.0.0.30 to a PC connected to its Ethernet port and to wireless client devices configured to use either no SSID or tsunami as the SSID, and with all security settings disabled. The mini-DHCP server feature is disabled automatically when you assign a static IP address to the 1100 series access point. Because it has a console port to simplify initial setup, the 1200 series access point does not become a DHCP server automatically."