The story of Hunter pets and aggro in Burning Crusade, especially since patch 2.4.2 or so, has been a very stormy one. Even deep Beastmastery Hunters have had to learn to kite, as pet aggro grows worse and worse. Growl in 2.4.2 was supposed to scale, but almost every Hunter that tested it said it didn't, despite Hortus' insistence that it did.
Luckily, it looks like there may finally be some real relief coming in Wrath of the Lich King.
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.