Not true. I adore Roberts and his extraordinary intelligence and he seems firmly grounded as a conservative (albeit not an activist like Scalia).
I was never enthusiastic about Miers; she came across as an Oprah-type. My only consolation is that she might have been a setup all along for someone more openly conservative, but the White House is horrible at politics. If they had much political sense, they would've gone for Janice Rogers Brown in the first place, given that she's got preapproval from the filibuster compromise.
The dems would fillibuster Brown, the question is whether or not there would be enough Republican support for killing the filibuster rule (the so-called "nuclear" option), my guess is that there wouldn't be. If Frist can't even muster enough votes to support Miers, how on earth will he muster enough to overturn the filibuster rules?
So that means that Brown/Luttig/Owens would all be DOA, there would be a big fight, but ultimately they would fail. I think the Bush team realizes this.