Agility currently gives you dodge so in 4.1 that neck is actually fairly decent for a warrior tank simply due the the large amount of mastery and reasonably high stamina. Haste would naturally be reforged to parry or dodge.
When 4.2 hits however, Agility becomes worthless to plate tank and the value of that neck will drop considerably.
Actually the best in slot is http://www.wowhead.com/item=60227 from Sinestra, however few tanks will be seeing that.
4.1 introduced http://www.wowhead.com/item=69635 in ZG which is an adequate strength neck for tanking. You can get higher mastery and stamina currently from raiding ones but the secondary stat would be hit or haste which is significantly lower value than dodge.
My general recommendation would be if you have the BoE, sell it. Cause when 4.2 goes live you won't want an agility neck anyway (it would still be decent but significantly weaker).
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.