I haven't had any luck with DHCP under OS X. Until my cable ISP agreed to give me a static address, I had to do this:
(1) Boot 9.1 under DHCP, note the IP assigned by the server
(2) Boot X, use Network Prefs to set that IP manually
(3) Rebot X using that manual IP.
Fortunately I didn't have to reboot often and have never had a crash, so this wasn't a big deal. Now that I have a static address it's no deal at all. No doubt the DHCP client will get fixed in some upgrade in the next few months. If you need it before then, do the above steps and leave your machine on (or sleeping) all the time.