802.11b equipment operates in 2.4 GHz with up to 13 different channels (frequencies), but not all of them are allowed worldwide. As it's already been said, it's an unlicenced band that can be freely used as long as emitted power does not exceed 100mW.
Average Wi-Fi equipment uses 30-50 mW of output power.
Re: 802.11g. It's not the fastest wireless available. It operates in the same 2.4 GHz band as 802.11b, and it should be capable of providing 20 Mbps. However, this figure drops dramatically when 802.11b client associates with a 802.11g access point, because the standard mandates that in this case some of the bandwidth must be allocated to slower client to "catch-up". The fastest wireless is 802.11a, which offers true 54 Mbps (not only declared one like 802.11g), but since it operates on 5 GHz band, the range is very very low.
Back on topic: IANAP (I am not a pilot), but I would consider any Wi-Fi equipment as a threat to filght safety.