If the Pismo has an Airport Card, this will be the original Airport Card (54b) and not the later Airport Extreme Card (54g). Therefore, make sure your router allows 54b connections - some modern routers are set to exclude the older, slower speed wireless cards. Or does the Pismo use a wireless network PCI card in the PCI slot on the side of the Mac? If so, check for updated drivers for that PCI card.
If you log-in to the router, there should be a section that shows the wireless speeds settings - make sure it's set to "b, g and n" - or if you have no "N" speed computers just stick to "b and g" speeds.
Also, if that doesn't work, try a new password on the wireless network and possibly a different encryption strength (WPA or even WEP, rather than WPA2 which might be be compatible with older Airport Cards.)
Last year, I bought a Belkin Wireless "N" speed router, despite it saying on the box that it was backward compatible with 54b and 54g, my Powerbook G4 could connect fine (with it's newer, faster Airport Extreme Card) but my older G4 iMac couldn't connect (with its older Original Airport Card), giving me the same error message as yours. In the end, the Belkin router went back and I got a refund, and I went back to my Netgear DG834GT which works with both Macs using the same wireless settings, encryption strength and passwords. So, all is not equal when it comes to routers it seems.
