I have an odd one:
I'm setting up my CCIE lab, I get my CUCM and CME working no issues with IPC.
Then I get to the second portion, add a CME with a couple of 7940's, they won't get DHCP? So I try everything and finally remove the switchport voice Vlan command and then they start working.
CDP is enabled, I can ping the CME / DHCP server and it's not on a subdot1Q because I got frustrated and flattend the network . Checked spanning tree (Or what passed as spanning tree on this switch).. Check all sorts.