Per yesterday's Pirates Notebook in the P-G, attendance has increased 21% over last year, up to 20,785 per game. And this in spite of a staggeringly wet and cool spring (although iirc the Pirates have had fairly good luck with homestandsª).
The big Phillies weekend helped, obviously, but, spread out over 26 home games, it's no more than a ~700/game bump
The bottom line is that, through what are typically their worst-drawing months, they're out-drawing their overall attendance average from last season. I've always contended that a good Pirates team would see an immediate increase in attendance, and I think that 2011 - which hasn't even seen a really remarkable quality of play or spot in the standings - is proving me right. The first season they're in a pennant race, they'll top 25k per game, mark my words.
ª Although I also recall a few April games where there were literally hundreds in the stands, more due to the Pens than the weather