pepsi@pepsi.com seems better than pepsi@pepsi.pepsi in my opinion.
At best, it would be pepsi@pepsi. (note trailing dot).
My buddy used to be root@ws. but only a few MUAs and MTAs could handle delivering mail there, and always with the trailing dot.
But if new TLDs become common I guess that will change with time.
For example how would the Google Chrome browser work (and search to a degree), does a user who types "pepsi" into their omnibar want to go to http://pepsi/ or search for pepsi?
It also adds confusion for webmasters trying to parses URLs on their website. When a user types pepsi.com/coke into their blog for example that link can be made clickable to http://pepsi.com/coke/ where it can't for pepsi/coke