While most multimedia solutions work with a few in a workgroup, getting the application to scale across the WAN is the costly and difficult part. At least today. To do a scalable, viable implementation of multimedia on the net, you need to ensure the quality of service, low latency and low jitter. Latency is an issue. The amalgamation of different latency times causes jitter, which users will recognize as pops and clicks in audio.
Routers add far more delay than switches.
Don't assume that ATM will solve all of your multimedia ills. It's early in the adoption curve. The IETF is also working on the Internet Group Membership Protocol, which allows machines to be a part of these groups. And what hits the local network spreads out to the enterprise. So be prepared.