My Blood Shield and my priests PW:S would like to have a word with you. Absorbs are an extension of your health pool. For as long as shields have been useful it has been this way. Every dmg reduce makes an absorb take less damage.
Want to check it yourself? Take any mob that hits hard enough to remove the full shield during it's uptime. If you don't find one, just head over to ICC and have a word with the first 2 trash mobs in there. Get PW:S and check how much damage it absorbs and how long it takes. Then do the same again but this time use IBF (+if available Bone Shield) and watch the damage it absorbs and how long it takes. Now get out there 'naked' get shielded and see how long the shield holds and how much damage it absorbs.
If this practical approach isn't your thing, go with the combat log. Just take a look at what it says how much damage is absorbed. If the hit would have been subtracted before mitigation there would be "absorbed BIG number" while if it's after mitigation you'd read "absorbed small number".
Why do you think a disc priests shield in PvP soacked as much attacks as it did? Because the damage was mitigated by resilience, If you'd get to pound on unmitigated shields, they'd be worthless compared to heals, since heals counter the mitigated damage...
I soloed AN, both instances,
4.0.3 (click for video) and 4.0.3a and in the latter my Bone Shield was about 10% less absorb as before while I took slightly more overall damage (no logs, numbers from recount).
The Blood Shield of a pre-mastery DK while soloing got reduced by 30% (10% -> 7%). So where it could take 1.25 hits before so uptime ~3-5s hit(absorbed)-dodge-hit(partially absorbed), it's now gone in 1 hit which it may possibly not even absorb completely. Blood Shield is really nice once you are not limited by the minimum heal anymore.