I realize that I'm resurrecting an older thread here, but I'm having the exact same problem with the latest Entourage and PocketMac 4.0 in dealing with tasks.
I sync my Calendar and Address Book via BES, but of course Entourage doesn't sync Tasks to Exchange, so the only way to get them onto the 'berry is via PocketMac and a USB cable.
I've given up on the two-way sync with PocketMac, as it is not reliable enough to trust. One-way sync isn't unreasonable, and at least allows me to carry my tasks around with me at least as efficiently as a piece of paper would work
I'm anxiously awaiting Mark/Space's Missing Sync for Blackberry, which I'm hoping will do a much better job.