also try ekahau heatmapper. You walk around with your laptop and plot signal reception on a pre-drawn floorplan. It then generates a heat map of signal strength. Very cool.
I have also used my itouch, I cant remember the application, but it was great at telling me info about the signal (direction . strength etc.), however it did not have a plotter. I would also presume it works on the iphone, but seeing as I dont have one I dont really know ...
netstumbler doesn't support my dell laptops wireless card so I found a little vista desktop gadget thingy from http://www.xirrus.com/
works fine for me.
will look into heatmapper, sounds funky