According to this 1UP piece, the 360 version of Left 4 Dead will have its own dedicated servers on Xbox Live, meaning congestion shouldn't really be an issue.
The news comes from the latest edition of the 1UP Show, which also bore the following info on the console SKU of Valve's upcoming zombie shooter:
- There are four campaigns, with five chapters each
- The hospital level seen in demos is based on Philadelphia
- Another level is based on Pittsburgh (evidently Valve has some Steelers fans)
- Other levels cover a small town and an airport
Loads more through the link.