Bosses no longer dropping rare?

Before 1.03 patch the bosses were guaranteed to drop at least 2 rares if you had 5 stacks of Nephalem Valor.

In patch 1.03, bosses AND elite packs are guaranteed to drop at least 1 rare if you have 5 stacks of Nephalem Valor. Otherwise, it is not guaranteed.

This was changed back to 2 rares per boss in a hotfix to 1.03 on June 28, 2012.

Bosses are now guaranteed to drop at least 1 Rare item for players with 4 stacks of Nephalem Valor and at least 2 Rare items for players with 5 stacks of Nephalem Valor

Source

With 5 stacks of Nephalem Valor you will still get at least one rare from a boss, with less than 5 there is no guarantee.

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.