Pet tanks are viable regardless of what some players may have to say about it, but you will have to have a player controllign the hunter and pet who is fast, dextrous, quick witted, not-prone to panic, experienced and willing to stand ground where other hunters pull back. Pet tanking is not easy, and its going to require you muster every peice of speed and thought you can, but it is certainly not an impossible task.
My hunter is a BM, and has had amazing success using pets as party tanks (needless to say much easier before the Heroic stage). But there are many factors to remember, and not the least of which is mentality of the group your with. It was stated earlier that your average PUG >pick up group< will have a tank who refuses to consider sharing the responsibility, and some get quite angry simply over the "growl" taunt of you pet being active. Many healers don't like the idea of healing pets for a hunter, and some people even refuse to buff the pets (simply idiotic on that point). Your TEAM must be willing to accept your pet as Tank or Off-Tank, because without the teams support your pet tank will get no where.
Secondly, your pet tanks stands no real chance of success at raised levels with the hunter as a MM or SV spec, the pet's bonuses are just too low, not to mention those extra four talent points for a BM pet make a world of difference in its capacity for any role.
Now for an odd bit to most hunters, Agility will no longer be your primary stack! WHAT?! Think tank, you need stamina and defense to survive and thats the tanks main job, survival where the others on the team would fall in a couple of hits if not one good crit. So now you need to start stacking Stamina, preferably from mail armor with good Armor Rating for highest carry over of defensive stats to your pet. Then we hit another oddity, AP bonuses on the hunters equipment become even more important...now hear me out on this. Your pet gets attack bonuses from your offensive stats as well, specifically your Attack Power. Now most hunters need that Agi first for all of its benefits beyond just AP, but AP pluses off of most items will stack FAR faster then the equivelent bonuses you get from Agi on items at the same ilevel. You look into the math for yourself but roughly 1 agi = 2 AP, so agi is still very important but you can more quickly stack AP onto your pet by paying closer attention to the AP vs AGI pluses on an item. So for tank pet, Sta then armor then AP/Agi but with more emphasis on total AP then total Agi.
And now a direct note to hunters who would like to try using a pet tank! Go practice!! The last thing any of us need is a hunter who says "My pets the tank for this go go!!" and promptly runs the team into party wipe hell due to under-tuned hunter skills or uncontrolled pet pulling in add after add. Now your practice will consist of some solo trials, against mob clusters as close to your level as you can find. Practice controlling your pet and its particular skills. First you need to practice how to grab aggro as the pet in a controlled fashion, and then how to hold it after the hunter starts shooting. Key skills to remember for this part are your pets special skills, growl, intimidate, kill command, call of the wild, deterence, disengage, frost traps, concussive shot, freeze trap(arrow), Heart of the Phoenix (if applicable), MEND PET MEND PET MEND PET, and the hunter staple Misdirection. These are the primary skills you will use while pet tanking to maintain pet aggro and keep the mobs herded towards the pet. Also at this time go pull the "Pet Attack Queue" macro from one of the macro threads here (seen it in a couple places now) and learn to love it.
Now presumably after hours of frustrating practice at this you may think you have it figured out, but you don't. Now you have to start practicing your hunter's shot rotations and personal DPS upkeep. You have to maintain all the things you learned previously but start stacking a new skill set on top, doing BIG damage without pulling aggro from your pet. Suggested rotation would be Misdirect(when available, and before your first attack on a new mob), kill shot(when available), serpent sting, arcane shot, multi-shot(if facing more then one mob, and not having major aggro issues in the encounter), and steady shot. Any experienced hunter will confirm that this rotation drops major damage and can be easily adjusted to fit into most fights.
Great now you know how to use the pet right, and how to manage a shot rotation without pulling too much threat (necessary hunter skill to have even when your not pet tanking). Now for stage three of your practice and training, kiting/crowd control while maintaining your tank at the same time. No other class in the game is quite as adept at this two jobs as a hunter, but remember your primary roll now is still tank, and hunter death now equals tank death to a party. Here you start in with PUGs, and use the pet as an off tank, and work on your kiting skills, keeping the adds away from your casters, pulling the occasional mob off the tank into a freeze trap to keep it out ot the way, and learning which hits you can and can't take to save your healer. This portion of your training is one of the most important for the longevity of your groups, even if you decide to pet tank you still need to keep the skills that make hunters so handy sharp.
So as you can see, being a pet tanker requires more skills then the average hunter, and lots of time to hone your reflexes. You will be mashing buttons with more fervor than ANY other class, and your going to develope a much more intricate knowledge of what your character and yourself are capable of.
Hope this helps any that are seriosly considering one of the hardest character concepts in the game, and if enough of you show interest I may even clean this up into a more detailed tutorial.