All tank classes gain armor from agi.
I just switched from 2H Frost tanking to DW Frost tanking and noticed my armor increased a little. When I keep all my gear the same, and just switch weapons, the weaps w/ stam/agi increase my armor over the one w/ stam/str.
I was under the impression that only druids gained armor from Agi, or have I been wrong this whole time and DK's (and other tank classes?) do too?