Okay in regards to Reforging for DK tanking, what I was told by one of the guys in one of the "top" guilds on my server was try to get Dodge to around 20% and Parry to around 19% no lower, and reforge everything else to Mastery to try to get 100% Mastery or more.
As to rotation that is a bit harder to say. You definately want to keep Frost Fever on the target(s) but as far as diseases that is the only one that is vital. Death Strike you only want to do maybe every other attack to get the most out of your "bubble."
I know alot of people had preached before to ignore Hit Rating.
But now it seems Hit Rating to cap and Expertise to cap are even more important than before, and your Hit Rating is extremely low so getting that up might help out.
General rule of thumb for gemming is to still stack Stam.
Come Cata threat should be alot easier for us since we will have the proper Runestrike, which means regaining our Runes alot easier.
Where are you having the most issues with threat?
Raids or Instances? If there are threat issues in a raid then need to inform the DPS/RL that they need to let Tanks establish threat. If it is in an instance, then let the DPS die, they are being stupid. The era of Instant HIGH Tank threat is gone.
Bosses or Trash Mobs? If the threat issue is on a boss, is it at the start of a fight? Then DPS needs to hold back a tad, again the era of Instant High Tank threat is gone, especially when certain DPS Specs/Classes can pull completely off a Tank at the start of a fight, which I myself have been guilty of a few times during the rare times I have been able to dps (Rime and KM procs on a Frost DK means big threat at the start of a fight). If the issue is on trash especially in an instance, unless the healer is pulling threat, I say they wanted it that bad, let them tank. DPS need to learn to use CC or smart DPS again, since the age of instant super high threat from tanks is gone.
But as for a "rotation" that kinda depends on what works best for you.
I'll admit I go slower now when tanking than before unless I know the DPS in the random I have show that they can handle themselves if they pull. But in general I try to open with D&D, Icy Touch, Pest, Blood Boil if the DPS is too eager, otherwise I try for D&D, IT, PS, Pest, BB, BB if at all possible.