That's fortunately incorrect. The radio will always speak appropriately to each individual client. However, overall bandwidth on the "g" network will drop drastically as "b" clients connect.
I've seen many different statistics on this, but the most common one I've seen is that you'll drop from ~28Mbps to ~14-18Mbps of actual throughput. Remember that, although a client connects at 54Mbps, once wireless overhead is removed it's closer to 28Mbps.
So the good news is that your "g" clients won't be talking at "b" rates, but the bad news is that they will all go much slower.