Exchange 5.5/Distibuted or Centralized?

I am looking for a definitive answer to the amount and type of WAN traffic Exchange 5.5 uses in these two models. THere seems to be differing answers to the question depending on whom you talk to.

Is there more or less traffic generated when all Exchange servers are located on the same LAN within a single high bandwdth office, serving satelite offices vs. Exchange servers located at larger satelite offices serving the end users in that office and surrounding “spoke” offices? Are thre any published papers on this subject?