Last time I shot for MTV (long time ago!) I just gave them the masters (BetaSP then). Generally, by my experience, it's best to let them worry about format conversion.
But yes, you might need to down-rez from 720 to SD. However, I think they might prefer HDV masters nowadays.
BTW What version of FCP are you using? I assume you have Compressor and QT Pro too?
In the U.S. I can't think of any broadcast outlets that would accept an HDV master. If it's SD, they want DBeta and if it's HD, they will want D5 or HDCAM. They are cheap too so they aren't going to want to pay for a format conversion when they can put in their delivery specs that you as the contractor must deliver on their preferred format.