5 DK's can definitely solo instances at the moment - Zero has even had some success in heroics. It's possible mainly becuase blood aura currently stacks, giving you effectively 20% of damage back in health. You'll need to spec at least 36 points into the Blood tree with these talents:
Rune Tap
Improved Rune Tap
Blood Aura
Vampiric Blood

You'll also need the glyph of Rune Tap as well as this makes your rune tap heal everyone in your party for 10% of their health, as well as healing you for about 50% of your health when popped with vampiric blood.

HOWEVER - Blood Aura is being changed in 3.1 and will most likely no longer stack, which will effectively kill off this build IMO. 4 Dk's and a healer should still be viable though.

I ran a few instances with my team on the way to 80 (mainly for quest rewards) and didn't have any major issues - most bosses you just have taunt on a round robin to spread the damage around, and use rune tap as required. The only really annyoing thing is having to run back when one of your toons gets killed.

Hope this helps...

Cheers,

S.