According to Ghostcrawler, they now realize that it's very easy for BC-era Hunters to pull aggro off their pets, and it's something that will be fixed in Wrath. Although they haven't done a full pass on threat generation yet, they want to make sure that a pet will be able to hold aggro against a Hunter in solo play. Even more so, a tanking pet should ideally be able to effectively offtank a 5-man instance and even main tank in some group circumstances, such as 5-man quests.
A preliminary look at both the Beastmastery tree and the Hunter pet Tenacity tree would certainly seem to bear this out. Beastmastery gets a Longevity talent which looks like it should reduce the cooldown of Growl. If it then stacks with Improved Growl in the Tenacity tree, that should make it a lot easier for tank pets to keep aggro, especially if you add the Taunt talent on top of that.
After a long Burning Crusade spent learning to kite, I know this news make me look forward to Wrath of the Lich King more than ever. It should provide Hunters both with an extra hook to grab 5-man groups, and make Beastmastery Hunters feel like effective soloers once again.