Cyril Morong does some significance testing on the number Bill James supplied on David Ortiz's clutch hitting. The only place Ortiz comes close to being significantly better in those situations is in extra-base hits:

Moving to XB%. He had a rate of 14.8% under normal circumstances while he a a 17.5% rate in the James clutch situations, for an 18% higher rate. The Z-score was 1.96 using the normal dropoff of about .013. So this is very close to being significant. His extrabase hit performance may truly be clutch.

If you think about it, that's what you really want in those situations, batters who can move runners a long distance with one hit. With a man on first, a single is nice, but an extra-base hit gives the runner a much higher probability of scoring.

Hat tip, BBTF.