Check out Paul's clutch stats on 82games:
06/07 season, 79.6% ft, 90.7% in clutch situations.
07/08 season, 84.3% ft, 83.3% in clutch situations.
07/08 playoffs, 80.2% ft, 88% in clutch situations.
08/09 season, 83% ft, 85.1% in clutch situations.
Seems pretty clutch to me.
To play devil's advocate, I think that in this argument, you have to compare apples to apples. The numbers you're citing to are mostly regular season "clutch" numbers, as defined by 82games.com. That's, what, the last 2 minutes of a game, with the game within 5 points?
What the original poster is talking about is something much narrower: Pierce's performance from the line in game-tying or game-winning situations in the playoffs. I'm not sure that regular season numbers taken from the last two minutes of games answers the question of how Pierce shoots his free throws.
As far as I know, there are no easily accessible stats for those situations (although there may be some regular season numbers to dig up). My guess -- and it's only a guess -- is that Pierce's career percentage in such situations is in line with his FT% otherwise. Anecdotally, it doesn't seem like he steps up his game from the line in playoff games, but I don't think he chokes, either. Rather, it's a situation where people remember the misses but not the makes.
I mean, here's the thing: Pierce is roughly an 80% free throw shooter for his career, which suggests that he's going to miss 1 out of every 5. If that one miss comes at the very end of a game, it's disappointing, but statistically isn't all that abnormal.
EDIT: See below. Looks like overall, Pierce has been outstanding from the line.