Really, I think everything should be about armor pen. It's just so fun to talk about. Poorly understood non-linear scaling stats with odd mechanics that are hard to model?
SIGN ME UP.
In the ongoing saga, you should definitely read Runy's take on armor pen, which has loads of good numbers, good suggestions for gearing, and awesome cartoons. As well as showing what you can do if you're going through ToC and have a dream list of awesomesauce.
But here's a more concrete setup: my current gear listing. In case armory isn't working well or isn't current, here's a chardev link too.
And the Rawr report:
Head Conqueror's Nightsong Headguard
Neck Collar of Unending Torment
Shoulders Duskstalker Pauldrons
Chest Vest of Shifting Shadows
Waist Relic Hunter's Cord
Legs Legwraps of the Broken Beast
Feet Runed Ironhide Boots
Wrist Armbands of the Wary Lookout
Hands Sunreaver Assassin's Gloves
Finger1 Dexterous Brightstone Ring
Finger2 Planestalker Band
Trinket1 Darkmoon Card: Greatness
Trinket2 Banner of Victory
Back Shawl of the Shattered Giant
Ranged Idol of Mutilation
And buffed stats:
Attack Power: 11276
Crit Rating: 459
Hit Rating: 241
Expertise Rating: 56
Haste Rating: 202
Armor Penetration Rating: 999
Crit Chance: 58.67081%
Note that unlike Runy's OMG loot list, this isn't close to being optimal. I have a few ToC 10 normal mode pieces (including the weapon), no T9 gear at all, a couple pieces from Ulduar (boots, head, belt), two fairly meh trinkets (and no Runestone), and a really crappy cloak and bracers (at least relatively).
ETA: In case you were specifically curious, the breakdown of that armor pen is as follows:
Gear (including socket bonuses) 677
That's just what's dropped for me. I haven't gone out of my way to grab nothing but armor pen gear. I picked up the shoulders because I haven't had a trophy piece yet (and would replace the head first, anyway). I have gotten very lucky on drops in ToC hard 10 man, being the only leather wearer and having almost exactly what I wanted drop.
But still, the important thing: it's gear that you can reasonably expect to obtain in the next month or so. No 258 pieces, no hard mode loot from Ulduar. Only 10-man hard mode gear. No trophy gear either. Hardest pieces come from heroic 10-man, which should be challenging but not insane.
Ironically, the upgrades I do want are going to drop my armor pen a bit. My next piece is going to hopefully be the T9.245 helm simply because with this level of armor pen missing a lot starts to hurt a lot more, and it has loads of expertise. A cloak upgrade would be stellar, but that's more luck than anything else; we're going for the Insanity 10-man tribute this week, so perhaps Cloak of Serrated Blades will drop. And who knows, maybe some day I'll actually get a wrist improvement, though most of them will drop my arpen too. The important thing here is that I'm fine with it dropping; it's not essential that it stays high compared to other stats. And while armor pen is scary good at this level (even after nerf), it's not so good that it will make up for 20 levels of itemization.
One thing that would significantly change my gearing strategy is getting a runestone. Even post-nerf I'm well over the armor pen cap with a runestone, so I'd be going for more agility. But I'm not betting on getting that either, and I'm not going to kill myself if I don't get it.
Anyway, point of all this was to toot my own kitty horn and to also show you that yes, you can get significantly high amounts of armor pen without having stupid gear.