Well here's the priority system dual wield frost dk's use!
1. If Killing machine has procced -- Frost Strike, if not then
2. If mob does not have debuff frost fever -- Icy touch, if it does then
3. If mob does not have debuff blood plague -- Plague Strike, if it does then
4. Do you have 1 x Frost and 1 x Blood Rune (or Death Rune/s) ready -- Obliterate, if not then
5. Do you have Blood Runes ready -- Blood strike, if not then
6. Do you have 32 or more runic Power -- Frost Strike, if not then
7. Has Rime Procced -- Howling Blast, if not then
8. Horn of Winter
The above priority applies to single targets. If there are multiple targets, you would do the following
2. If target mob does not have debuff frost fever -- Icy touch, if it does then
3. If target mob does not have debuff blood plague -- Plague Strike, if it does then
4. If target mob has both debuffs and pestilence is available -- pestilence
5. Has Rime Procced -- Howling Blast, if not then
6. Is Howling Blast off CD -- Howling Blast, if not then
7. Do you have 1 x Frost and 1 x Blood Rune (or Death Rune/s) ready -- Obliterate, if not then
8. Do you have Blood Runes ready -- Blood strike, if not then
9. Do you have 32 or more runic Power -- Frost Strike, if not then
10. Horn of Winter
11. Fill any free GCD's with Blood Boil.
On bosses the rotation is the same as the first priority EXCEPT where and when you get a KM proc and a RIME proc together you Howling blast. There is also the topic of where and when to use unbreakable armor but i'm still looking into it atm, there's lots of debate.