Hit "soft" cap is 8% (actually 8,02 I think I read somewhere); my understanding is that the "hard" cap concern only dual weilder. Anyway the soft cap is what you need as a DK (unless you dual weild of course, then it something like 26%, for white swing).
Expertise "soft" cap or dodge cap is 26, or 6,5% and make you ignore all dodge (from bosses) but not all parry; the parry cap is unknow because it depend on the boss (I think it vary from 10 to 15%)
If want more informations you can check either here: Combat rating system - WoWWiki - Your guide to the World of Warcraft
or here: [FAQ]Working theories of raiding at level 80 - Elitist Jerks