I would add four additional points to the discussion of potential opposition to John Roberts:
1) Roberts is an outstanding lawyer, excellent judge, and impressive individual. He is the sort of person, irrespective of ideology, who President's should nominate to the High Court (and those who have worked with him generally feel this way, even when they disagree profoundly with his legal views).
2) Roberts was initially nominated to the D.C. Circuit by the first President Bush, and the Democratic Senate refused to move the nomination. Some believe this was due to concern for the overall "balance" of the D.C. Circuit, or perhaps it languished as nominations often due in the last year or so of a President's term. Either way, he was nominated back then, and never confirmed.
3) Activist groups have already begun to misrepresent John Roberts record on various cases. For instance, he has been accused of voting to overturn the Endangered Species Act. In actuality, Roberts dissented from a denial of en banc review because, he stated, the panel decision was in conflict with those of other circuits. His opinion did not challenge the panels conclusion. Rather, he argued that there were reasons to be concerened about the conflicting (indeed, incompatible) rationales adopted by different circuits, and that this merited en banc review.
4) Last, and most distressingly, the leaders of various liberal activist groups are already on record stating that they would urge a filibuster of Roberts, even if he were nominated to replace Rehnquist. While I doubt (hope) this would not derail his confirmation, I suspect it would sway at least a few Senators.