The contents of this post are from datamining the PTR patch from today.
Blizz changes PTR patch contents all of the time; this may not be the final state.
Normal gear from 10 and 25 Ulduar has a +6 ilevel buff.
Hard mode gear from 10 and 25 Ulduar has a +13 ilevel buff.
Arena gear has received the same +13, crafted BoEs for the phase are +6.
Hard mode drops the same amount of normal ilevel gear as normal plus 1 piece of hard mode gear at hard mode ilevel from a pool of 3-5 (unique to boss).
A 1 ilevel difference is approximately 1% more stats. Weapon damage and armor use a slightly different formula. As an example, Voldrethar, Dark Blade of Oblivion is a hard mode weapon which normally is 239 ilevel. Here are the stats for that weapon at 252 ilevel after the buff.
Voldrethar, Dark Blade of Oblivion
775 - 1163 damage, 254 DPS
148 str
165 stam
98 crit, 79 arp