"Note that a 5-CP Expose Armor with 2 points in Improved Expose Armor is worth slightly more armor reduction than Sunder Armor (3075 vs. 2600). If your tank is a protection warrior, then Expose Armor will overwrite and prevent the use of Sunder Armor. "
Improved Expose Armor does not replace Sunder armor
From http://web.archive.org/web/20080818023903/http://elitistjerks.com/f47/t24301-rogue_pve_dps/#post844053#Raid-wide_Buffs_and_Boss_Debuffs
"Note that a 5-CP Expose Armor with 2 points in Improved Expose Armor is worth slightly more armor reduction than Sunder Armor (3075 vs. 2600). If your tank is a protection warrior, then Expose Armor will overwrite and prevent the use of Sunder Armor. "
I made a couple of screenshots to prove it : http://imgur.com/XGkFM1Z (with 5 CP, IEA 2/2, 5/5 Sunder Armor) http://imgur.com/i0CRmyb (with 5 CP, IEA 2/2, 3/5 Sunder Armor)
"A more powerful spell is already active."