Last Post: April 1, 2010:
If I understand your question correctly, then part of the answer is - it depends.
From what I understand, the dot11 spec only defined IBSS for "b" mode, so it would depend on the chip set and the driver in that case.
I have several PCMCIA b/g cards that can be set to run either "b" or "g" in Adhoc mode, but most of my older cards don't even have an option for this and are "b" mode only.
Most people don't know it, but the Japanese channel 14 is not only out of step frequency wise with the other channels, but is also officially only a "b" rate channel.