David,
You have probably seen
this already, but I post it just in case.
It does make mention of disabling the "G" side of things, so that could be worth trying. Also what are you using for encryption, Nothing/WEP/WPA/WPA2? It seems to suggest going to WPA/WPA2, so if you're not using that, try it. Thing is, some things might need WEP, I recall reading something that said Blackberries use WEP, so if you have one this might not be an option.
Pretty sure you have read
this on Apple's discussions forum. One interesting thing is the mention of Open vs WEP vs WPA, in that open = ok, WEP = problems and WPA = ok. May be something there as mentioned above.
Another mention was to use opendns' DNS server addresses (208.67.222.222 and 208.67.220.220). Not too sure where you'd put these in, and I mean at a device level, wether it should be your laptop, or the router/repeater (replacing or supplementing whatever DNS server info is there). I'd try your laptop DNS settings first, as it won't mess with the rest of the functioning network setup.