One big problem is when you aggregate both onto a single, small pipe. The main concern is QoS for voice. One common solution is using ATM and multiple PVCs (this is what I am doing in the lab to demonstrate the QoS nature of ATM). Another is to have a separate physical path for voice and for data.
Many data LANs are not ready for voice (and neither are the network engineers). Voice requires a fully switched LAN (No hubs) and some diligent work to root out duplex mismatches. You also need to educate the network engineers on the sensitivity to network outages (many engineers don't mind rebooting a router).
Here is one link from Cisco about getting ready for VoIP. http://www.cisco.com/univercd/cc/td/doc/product/voice/ip_tele/solution/4_design.htm
