I think Nate Silver is exactly correct that Mel Martinez's resignation is nothing but a headache for Christ. There's really no upside here that I can see. A legislative (voting) record is always a great source for attack ads, and now Christ will be exposed to attacks based on the replacement's votes.
I don't agree at all that a Christ-appointed placeholder is more likely than Martinez would have been to vote with the Democrats on health care. Retiring Senators are wild cards; Martinez might choose to vote on what he sees as the merits, or he might try to cut a deal for something he cares about (including post-Senate employment or perks). Both of those could potentially yield a yes vote. Certainly, by voting for Sotomayor, Martinez showed that party discipline means little to him at this point.
Christ would be nuts, however, to appoint someone who would vote for health care. It would be a major weapon against him in the primary, where he would be forced to differentiate himself from the person he appointed, and having done that the vote would be no asset to him in November.
So this is mild good news for the DSCC and for the chances of the Democrats remaining at or over sixty Senators in 2011, but bad news for passage of health care reform.