He asked people on Blackberry forums whether they'd trade their Blackberries for iPhones.
I'm sure that gave him a representative sample.
The point of the post is whether or not the iPhone is ready for enterprise use. Enterprise users are mostly BlackBerry users. Who else would you ask if they'd make the switch or not?
Enterprise IT departments, regardless of what they use (especially if they use neither!). By asking people on Blackberry-specific forums, he's targeting people who have already made an investment in some Blackberries.