I am proud to say that my Voidwalker, Konkath (or KK for short) has main tanked the following Burning Crusade Instances:
-Hellfire Blood Furnace (level 73)
-Old Hillsbrad (level 69)
-Underbog (level 70)
In Blood Furnace, KK main tanked along side another warlock's Voidwalker. Old Hillsbrad is an excellent instance for a Voidwalker tank because Thrall is there to tank most of it. For Underbog, I was just a higher level than everyone else. My guildies had PuG'ed in a ret Pally to tank, but that wasn't working at all, so I came in and told the group that KK would be tanking, and told the Pally to concentrate on DPS. Then I asked the healer if things were going better, and she said it was.
Voidwalker tanking in instances is a little different than soloing.
Voidwalker Tanking While Soloing:
For melee attackers, just send in the Voidwalker, let him taunt the mob, then start dotting it up.
If there are several mobs in a big group, you can either 1) take them all out at once, dotting them all up and then using AoE and/or fear, or 2) pull one with a shadowbolt, sent Voidwalker to attack who will then taunt the mob, but you'll also need him to cast suffering. I like to take suffering off auto-cast in these circumstances.
For casters, again if they are alone you can just send in your Voidwalker and let him at it. Otherwise, you'll need to do what is called a "line of sight" (LoS) pull where you shadowbolt the mob (have your Voidwalker on "passive"), and then run around a corner if you can, or if not just run back really far out of the caster's line of sight or range. Then send the Voidwalker in, and you may need to use Suffering.
Voidwalker Tanking in Instances:
Since the Voidwalker can't take as many hits as regular warrior, pally, and druid bear tanks, you'll want to make sure you can get CC on most of the other mobs or use an off-tank (a ret pally, fury warrior, and hunter's bear pet can work in this role). Get the party leader to make you leader or convince them to mark all the mobs as to who will tank or CC what and let everyone know what order it should go in. Whatever your Voidwalker is tanking should be killed first. If there is an off-tank, that should be killed second, but try and get your Voidwalker to grab aggro from the offtank using suffering. You also may need to use suffering if someone else grabs aggro from the Voidwalker. If you grab the aggro, you should save the suffering cooldown and use soulshatter on yourself. If there is a pally in your group, get them to cast blessing of salvation on everyone but your Voidwalker and the offtank--this will help immensely. Finally, if your Voidwalker is about to die and your healer is out of mana, sacrifice him! Then drain tank and hope the healer can gain enough mana to keep you alive.
Final note: Using your Voidwalker as a main tank is really hard and should only be done if there are no better tanks available. You should also try to stick to instances that are a bit lower level than you are so the bosses wont seem to hit so hard. It's best used for helping a bunch of low-to-mid 60 friends be able to go through instances when otherwise no tank is available.