There's NO reason for them to abstain. Roberts had to abstain in Hamdan v. Rumseld because he had WRITTEN the opinion on the circuit court, that's a clear conflict. Neither Alito or Roberts have ruled or taken part in the NSA deliberations and therefore do NOT have to recuse themselves.
Yeah no reason to think that Roberts or Alito will recuce themselves, they never had any part of any court case dealing with this. And they'll vote to overturn it, but I don't think it will get to them. I think the appeals court will oberturn it. Five other Judges in the U.S. have already said that the NSA plan is legal and Constitutional. That strong establishment of precedent will rule the day.