the casing on the powerbook is what limits the rang of the airport. If it is a Ti book you notice the antennas are actually located near the keyboard on the sides.
I saw this one mod, I thought that it was on macmod, the guy ran an antenna out of the IR port on the back. But I know there are some really cool after market antennas that attach right to the powerbook and add alot of reception.
"Fix it 'til it Breaks."

Jacob - EiC & Director of Technology
Mac Pro Quad 2.66 - 4GB RAM
160 GB SATA RAID 1 - 650 GB Storage
Quad 19" Widescreen LCDs
Accessorized to the Hilt