Medicare Medical Savings Account (MSA) Expansion Act of 2003 - Amends title XVIII (Medicare) of the Social Security Act (SSA) to repeal the sunset date for, and numerical limitation on, Medicare participation in Medicare+Choice medical savings account (MSA) plans under Medicare part C (Medicare+Choice). (Thus terminates the demonstration basis for MSA plans, making authority for them permanent, and the number of enrollees without a ceiling).