Sorry, I don't really play a DK. So I can only tell you some overall values.
Melee hit cap for special attacks (thats all the buttons you press) is 6%. If you are using 2 weapons (what frost probably does) you will need 27% hit to never miss an auto attack. (The expertise soft cap to not getting dodges any longer is 26, but thats the same for all melee and also true for special abilities.)
While there is a theoretical crit cap, nobody can reach it with actual gear. And probably nobody will be able to ever hit it anytime during Cata. However I don't know if there is a special crit value for frost DKs that "should" be reached to get something going or whatever. I know that fire mages have such a soft cap, but I don't what's going on for DKs.