It's a little late for this particular conversation, but for future use, netstumbler is very useable for this: http://www.netstumbler.com
. The effects of antenna pattern are very clear when using netstumbler.
In addition, here is a link to a presentation on the North Texas Microwave Society site discussing the use of the Proxim Rangelan 7400 pcmcia card as a limited functionality spectrum analyzer. This card runs around $30 on eBay.http://www.ntms.org/files/
The Proxim Rangelan 7400 is an obsolete (pre 802.11b) wireless network card that did frequency hopping in the 2.4GHz band. The software for the card includes a signal strength scanning function for the 80 used channels in the band.