[Tunnelrunner]

Basically what Reyza said.

-The Bold is a "business device" that has multimedia capability, while the iPhone is a "multimedia device" that is (I'm told) slowwwwly gaining acceptance in the business world.
-If e-mail/productivity are most important to you, the Bold (the most beautiful device this side of the iPhone) is your best bet. The iPhone does e-mail too, but come on: berries are still the king. On the other hand, the iPhone is supreme as far as music (iTunes) and web browsing go (I've tried the Bold's browser; it doesn't compare to the iPhone, but who does?).
-Both 3G powerhouse phones suck a LOT of battery.
-Both are incredibly versatile. I'm guessing the Bold has superior PIM features.

It's a tough one. When I upgraded from a Treo 700p, these were the 2 phones I narrowed it down to: IMO, these 2 phones are overall, the very best that the mobile world has to offer right now. And they're both backed/supported by the 2 biggest superpowers of the smartphone world: Apple and RIMM. It was a tough decision, but I did go with the iPhone--I don't work in the corporate world and music and web browsing are important to me for everyday living reasons. Also, the iPhone gets updates faster and more efficiently than anyone else. RIMM was the king for years, but that seems to be changing with Apple's growing dominance.
-The bottom line is that you really can't go wrong either way. Both the Bold and the iPhone 3G represent the best of the best right now. Just don't buy a Treo/Centro...