I keep reading that it was amazing, fun, and completely unbalanced. But nowhere can I find the actual rules for it.
The current one in the character builder on the Wizards site is pretty unimpressive. It's at the bottom of the page here.
From Dragon #375
Oso the Circus Bear
Speed 4, fly 6 (hover)
Constant Benefits: you gain a +2 bonus on Intimidate checks.
Active Benefits: Oso can speak any language you know, and can converse with other creatures. Unlike mere familiars, you can’t hear everything Oso hears and says unless Oso lets you, and you would never be able to tell Oso what to say.
As a minor action when Oso is in active mode, you can place a curse on one target in a square adjacent to Oso. Until the end of the encounter, your attacks deal an extra 1d6 damage to that target, exactly as the warlock’s curse ability on page 131 of the Players’ Handbook. at 11th level, you deal an extra 2d6 damage to a cursed target, and at 21st level, you deal an extra 3d6 damage.
Oso deals 1d10 damage to anyone who attacks him and misses. at 11th level, he deals 2d10 damage, and at 21st level, he deals 3d10 damage.