I agree with the above poster. The match is a binding commitment on both parties. Most importantly, the ONLY party that can approve a waiver is the NRMP. Hence, if you call your future fellowship PD and they say "no problem, I release you from your contract", that is NOT a waiver. The NRMP must approve it.
It's unclear if the NRMP will approve a waiver for a change of heart, and either way the fellowship PD is sure to find out. Remember that the fellowship lost out on their next choice because you matched, so withdrawing now really leaves them in a hole.
Your best option(s) are these:
1. Live with it for one year, then resign; or
2. Contact your future fellowship PD and explain your situation. If they support your withdrawal, that can only help your case with the NRMP. If they don't, I doubt the NRMP will grant a waiver. You would then petition the NRMP and see what happens.
Take a look here:
http://www.nrmp.org/fellow/case.html
It appears that if you give them at least 9 months of notice, you might get a waiver. If you were planning on starting July 2007, that's right now!
3. Talk to your current PD for advice - usually a good idea.
PS If it's a cardiology spot, send me a private message. I have a PGY-3 who would like a spot!