As I understand it RIM have done some specific enhancement with GMail to facilitate this. Longer term I think they'll come up with a method to allow instant delivery on all POP3 accounts (hoping!)
Posted from my CrackBerry at wapforums.crackberry.com
I just believe it's the contract that GMail / Google have with RIM, same with Yahoo...
I'm just happy they offer it. Plus, I think people would be quick to get a 'free' email service with GMail for instant email on a BlackBerry rather than some 'other' POP3 type account...