Do bosses drop a higher chance of iLv63 items?

Blizzard want you to farm elite packs rather than bosses and have adjusted the guaranteed drops from bosses and elite packs in the 1.0.3 patch:

The Nephalem Difference

It’s no secret that our goal for the end-game item hunt is players hunting monsters packs, building to five stacks of Nephalem Valor, and then killing a boss. While we’re seeing a lot of that occurring, what we’re missing is people feeling like it’s worthwhile to continue onward after killing a boss.

To help hit that goal we’re lowering the number of guaranteed Rare items on bosses when you have your full five stacks of Nephalem Valor from two guaranteed Rares to one guaranteed Rare (you still have a very good chance at multiple rares, it's just no longer guaranteed). In exchange, all champion and rare packs will now drop a bonus guaranteed Rare item when you have your full five stacks of Nephalem Valor. The change benefits players with more overall drops, and a reason to push to continue progressing.

The bosses do not have a any higher percentage to drop higher item level then the other rate champion/elite monsters. According to the developers what they want you to do is start an act and play though it. Not just do boss runs over and over again.