A Couple of Things

Ran Shattered Halls with the guild today.

Some things to mention.

Vox generally kicked my ass with damage dealt, beating me by over 200k damage.

We had a paladin tank running the first part. While not in the guild, myles is an alt of someone who is.
And we were brilliant. The ridiculously hard pulls generally went good, for what was essentially a PuG. 2 mages, paladin tank, enhance shaman, and holy priest on heals.
We wiped twice. Once in a pull where we accidentally pulled 4 additional mobs, and once on the last boss.

Now, I'm going to go into detail over arcane versus frost here, instance wise.

Arcane had more damage. Substantially more.
At the end of the instance, I had done 800k damage, and foxy voxy had just broke the 1 million mark. This shouldn't come as a surprise.

Frost doesn't take a lot of hurt. On Nethekurse, when he does that impressive spinny thing, the deathcoil bolts dug in a lot more on Vox. I thanked the frozen gods and continued casting, grinning behind my Ice Barrier. Meanwhile, over voice-chat, Vox sat their cursing. *evil grin*

When AoEs were called for, frost nova + cone of cold unleashed a world of hurt on some orcs.
There is a great deal of satisfaction to be gained from watching 6 orcs take 1500+ damage each from a single spell. Even better if you can instruct your arcane-missile-happy friend to blow frost nova eight seconds later. *evil grin*

Random tip: you must MUST must be able to get a shatter combo on any single target in an instance. Any time you see a frozen target that you can ice lance, DO IT! 124 mana for 2k damage? Yes please! It's a huge source of damage for a frost mage in an instance. This is why you should always get "Add" duty.

As a good example, take Bladefist. Adds show up in this fight, in the form of silly non-elite orcs.
These can be a real hassle for the healers, even more so if you only have one. Like we did.

Enter me, the frost mage. The first time we hit Bladefist, it went horribly wrong from the beginning. Three (3!) assassins patrolled right into our group as soon as the fight started.
So, here I was. On add duty. I thought I had to deal with these uber-powered, incredibly hard hitting orcs in waves of three. Our group bit the dust. HARD.

Second try, no pats. Just the regular, non-elite, non-70 adds. And they come one at a time.
I almost cried, my job was so easy. Frostbolt kite, root, shatter kill. NEXT.
Of course, when Fistblade went into "ZOMG" mode, things got a little more hectic. The combination of losing frostbolt time and Vox screaming "HOLY SHIT!" led to a few accidents.
(Vox doesn't like being smacked around, you see. Seeing multiple hits of 1k on his screen scared him. /cheer for Ice Barrier.)
One add got on the healer, and she took a couple of hits before I snagged aggro.
A couple adds ran into the paladin's consecrate... that was harder to get aggro back for, but a fireblast did it.
And other than that, most of the adds were flawlessly slaughtered.

So, in answer to the question, "Is frost viable in end-game PvE" the answer is definitely yes.
But taken in moderation. If you're doing, say, Karazhan, only one mage need be frost. Any more and the party starts getting a little gimped.
Our jobs are easy. Incredible crowd control, strong dps (and more sustained than other mage specs, I might add), add duty, and kiting. We can root mobs, kite mobs, sheep mobs, and unload incredible burst on frozen targets.
As mages, we also have access to powerful AoE spells, and as frost, we add CONTROL to that high volume of damage. For example:
Arcane Explosion = AoE damage
Cone of Cold = AoE damage, can be aimed more accurately than AE, crits very hard, adds a slow to all targets, and has a 15% chance of rooting all targets. And cheaper, too.

On top of that, we have far better mana efficiency than the other specs, having mana to use long after the other specs have gone OOM.
Oh, and Ice Barrier, which makes the healers job that much easier.
And Ice Block, basically an instant aggro dump.

Sounds a lot like a hunter, don't it?
If you want to look at it that way, Frost mages are the hunters of spell-casters. Played right, we offer a lot to a group. Played wrong, and you get a Blood Elf casting Ice Lance because it looks cool.

Remember, my children, the creed of frost mages, as laid down by the prophet Faxmonkey:
"If you can chill it, you can kill it."

To all you PvE whinos : Shut your trap. Frost is great for Instances.

Random fun story:

Mylesmason, the pally tank, had to go to a birthday party with his kid (Aww.... <3) so we had to PuG a new tank. Snagged another pally tank and... well, lemme put it this way. Myles was miles beyond this guy in the tanking department. Please don't slap me.
It was laughably easy to pull aggro.
On the second boss, Lost Cause's GM (the enhance shaman) pulled aggro... and kept it. Enhance shaman with spirit weapons, Blessing of Salvation, versus paladin tank with Righteous Fury up.
Enhance shaman kept the aggro the whole fight.
Now, what's awesome is that Oli had a shield, (a good one, actually) and busted it out.

And tanked.

You heard me, a shaman tanked Warbringer O'mrogg.

Sadly, no-one got a screenshot...