Besides well-thought-out minimal kernel configuration, as with most embedded Linux projects the key to getting everything to fit is to link userspace applications with uClibc and to use busybox to stand in for any applications of which it is capable. cramfs makes it possible to fit everything in the memory.
What makes OpenAP even more interesting is the ability to use it in a meshing setup, with either bridging (802.1d Spanning Tree Protocol) or routing on AP-to-AP links. This in turn is enabled by the WDS (AP-to-AP linking) capability of the Host AP driver for Intersil Prism based 802.11b cards. This kind of meshing capability is currently found on no consumer 802.11 equipment.
Original work on OpenAP was done at instant802.com but since then various developers have continued in different directions. Local work on OpenAP is found at http://tools.collegeterrace.net/openap-ct/.