Just called around to find out the latest mileage offers for business service:
AT&T: The only promo is 5000 Delta miles one time only for signing up for service. You must mention the promo from Delta's Communications Partners web page. No miles are awarded for dollars spent.
MCI: Northwest is the only airline partner. 5 miles per dollar spent. Their service is the least expensive of the three. Their bundled local and long distance service is limited to 15 lines and 2 hunt groups per location, and does not support Centrex. If you need Centrex, you must sign up with another local provider (including MCI's separately run, non-mileage awarding Worldcom division), and use MCI's long distance service only.
Sprint: United is the only airline partner for business service. For Sprint Complete Sense, the bundled local & long distance package, they offer 250 miles per month for Basic, 500 miles/month for Unlimited, and 750 miles/month for Premium. There are restrictions about number of lines per location. They do not offer Centrex service. Their bundled service is the costliest of the three.
MCI was the winner for me, despite having wrestled with horrible customer service over the years. I miss their Southwest Airlines relationship. As of today, however, Southwest has not yet partnered with another long distance company.