Took me a while to find this so I thought it was worth a post.
We use the BlackBerry Enterprise Server to redirect emails to our BlackBerries. I wanted to be able to access my personal email accounts on the same device.
Conceptual leap for me, apologies if it was obvious, was that I needed to setup the BlackBerry Internet Service with my carrier.
If you're on Vodafone UK then you go to mobileemail.vodafone.net enter your PIN and IMEI and you're away.