Haste vs Armor Penetration.  Which stat is better for Ret Paladins?  There is a alot of argument and confusion over which stat seems to be better for a Ret Pally.  You can make very solid cases either way, but nothing is actually set in stone.  There ahave even been some conflicting comments here on my site as well.

After a couple weeks of research, playing around with Rawr, checking the EJ, WoW, and retpaladin.com forums, it would appear the general concensus is that Haste is still better than Armor Pen, even with T10 2-piece bonus.

Armor Pen only affects Crusader Strike, Divine Storm, and normal melee hits.  Haste affects weapon speed, generating more white hits, ultimately equalling more DPS.  Keep in mind that the T10 2 piece bonus procs off normal melee swings, and the more haste a ret pally has, the more chances DS has to proc.

The EJ BiS thread has discussions of haste even competing with crit as the primary stat behind strength on single target bosses with 4 piece T10.  While in theory this may mathematically work out in some scenarios, accounting for human error and movement during combat should negate this until further testing can be done. However, there is no mention of armor penetration even coming remotely close as a stat to reckon with.

So, as you progress through ICC, strength, crit, and haste gear still tops all.  You will still gem for crit over haste in yellow slots with strength bonuses.  Hope this helps, feel free to comment below with any other observations!