Over at the Mahablog, Barbara says:
Bush (or, more likely Rove, who must've had a hand in choosing Roberts) wants a nasty, headline-grabbing fight. The messier, the better. As Chris Bowers says, the White House is using one partisan hack to deflect attention from another.
I don't know if Bush will get the distraction he wants. On the whole, Dems in Congress remain calm about the Roberts appointment.
I'd take that a step further: Bush and Rove must know that they're going to get less of a fight than they could have -- for some reason they decided not to go for the most polarizing nominee possible, even though doing so would have driven Rovegate off the front page day after day after day.
What's going on? I think Bush is feeling a bit emasculated by his inability to get John Bolton confirmed, and perhaps by the filibusters (and the deal that didn't really make the filibuster threat go away). Maybe he's accepted the reality that Democrats can sometimes bite back, and he wants a nominee who can be confirmed, just to prove to the world that he's still effective, that he's still The Man.
Or maybe this is jiujitsu: Name someone who's a bit to the left of knuckledragger, hope the Dems fight back very hard, then attack them as unreasonably partisan -- and carry that charge over to the Rove story. But that only works if the Dems fight to death, and it looks as if they won't (see Barbara's post).
Or maybe the timing of the announcement was a tactic, but the choice wasn't -- maybe Bush sees the restocking of Supreme Court as his legacy, dammit,* and (after limiting himself to candidates deemed ideologically correct by the religious right) he went with the corporatist/partisan hack who just felt right to him. I do sometimes think that he's not really an ideological brawler -- he just believes in religion and deregulation and kicking evildoer ass, and he can't understand why everyone doesn't just agree.
*I probably should have said "part of his legacy, dammit" -- see what Aimai says in comments (and my reply).