I think the itemization on the T9 and T10 pieces are identical, so it really doesn't matter too much which pieces you pick up. Ultimately I'm planning on this though:
T10 Head, Shoulders (I'm sick of seeing the T9 shoulders and the T2 throwback helm)
T9 Legs / Chest (I don't need any extra expertise so I really don't need to grab T10 legs).
Frost Gloves: Gatecrasher's Gauntlets - Item - World of Warcraft (Really sexy itemization, sockets, and price.)
I think I would mix and match depending on your current gearing situation, but I don't need anymore Hit, so the T10 gauntlets are out of the picture for me.
T10 shoulders will be my first purchase, as they're very cheap. Then I'm going to grab the T10 head, so I can get the T9 2 set and T10 2 set bonuses together. Then gloves (or possibly cloak if nothing drops for me in the next 2 or 3 months). After I've run through all the Frost offset purchases (belt, gloves, cloak, and throwing weapon and maybe trinket) I'll think about replacing the T9 stuff and going for the 4 set T10 bonus as I'll likely not need anymore crit. But that's way in the future and probably wont happen before the next expac drops.
Has there been any research done on the 4 piece bonus? I have been wondering what kind of dps output we could see from slam double proccing.
Anyway, Ill be looking at T10 helm and shoulders to start with, getting offset chest, gloves and legs. I agree with skyborn and already picked up those gloves, as ive done some tooling around with chardev.org (a great gear design site if you havent checked it out) and created a arp capped gear set with the new icc loot.