The standard for g was written so that b would still be able to access it. As you pointed out, you aren't going to get 54 MBps, but they will work together.
There are two modes for 802.11g - compatible with 802.11b, or strict 802.11g mode. In the compatibility mode, an 802.11g access point has to fall back to 802.11b speed if there are any 802.11b components in the local "air space."
Agreed that I should have mentioned compatible and strict mode. Still, I stick to my point.
