I dual spec with Unholy for DPS (gear not currently on for DPS), and I'm a frost tank ATM and will be switching to blood for cata. But until then, am I doing ok? I've been told I'm too squishy and just upgraded to some cobalt armor and not sure if that was the smartest move or not. I haven't run any instances as a tank since I got my gear, but before I do I was hoping I could get some feedback. If you have any additional questions for me, let me know.
Try this spec out http://www.wowhead.com/talent#j0xZhxxxA0I0c0buzAo0x.
As far as being squishy, the spec will help out a ton as well as getting more gear with defense and enchanting for defense.
Try to reach your def "cap" for your level which i do believe is calculate as ((your level) *5)+135 for mobs 3 lvls above you. for mobs higher than 3 lvls above you add 5 more defense for each lvl.