From the terms and conditions on their web-site:
"Existing MCI WorldCom customers not eligible for bonus offer. Limit one enrollment bonus per AAdvantage account number and MCI WorldCom account. The names on your MCI WorldCom and AAdvantage accounts must match to participate in this program and earn AAdvantage mileage. Enrollment in the MCI/AAdvantage program is in lieu of any other MCI WorldCom bonus program or promotion. MCI WorldCom reserves the right to amend its mileage program upon 6 months notice. Allow 8 weeks for bonus miles to post to your AAdvantage account. You must remain an MCI WorldCom customer through time of fulfillment to receive full bonus. Miles earned are subject to U.S. Federal Excise Tax.
Please allow 3-4 weeks to receive your MCI/AAdvantage Calling Card.
American Airlines and AAdvantage are registered trademarks of American Airlines, Inc. American Airlines reserves the right to change AAdvantage program rules, regulations, travel awards and special offers at any time without notice, and to end the AAdvantage program with six months notice. Any such changes may affect your ability to use the awards or mileage credits that you have accumulated. American Airlines is not responsible for products or services offered by other participating companies. For complete details about the AAdvantage program, visit
www.aa.com. "