Yes I have both 802.11b and 802.11g (regular and extreme) devices on my network.
One oddity though is my 802.11g clients (extreme) kinda have a tendancy to puke when my TiVo's (802.11b) are copying a show from one to the other.
Other 802.11b devices are fine, just the 802.11g (Powerbook 12") seem to get upset about it.
I have an iMac with Airport, Powerbook with Airport Extreme, two TiVos with USB 802.11b Linksys, and my work laptop (IBM) with 802.11b in it.
There is an 802.11b/g compat mode on the Extreme base stations. You can force it all g or all b if you want as well.