With a bladder build, a molded, rubber bladder will want to return to it's natural shape, if not affected by either the air pressure or vacuum found behind the bladder.

Some manufacturers are now providing shock caps that allow you to build your shock under multiple configurations:

-Bladder with a Vented Cap
-Bladder with a Non-Vented Cap
-No Bladder - Emulsion with a Bleeder Screw in the Cap

Bladder with Vented Cap (openly drilled hole in cap, no screw)
With the bladder builds, if you remove the closed in cap from the equation, the elasticity of the rubber bladder and how far it needs to stretch (based on the volume compensation as the shaft is pushed in) will determine how much it (the bladder) will want to recede back to its original shape, after releasing the shock shaft. As the bladder tries to recede back to its original shape, it will pressurize the oil chamber, forcing the shock shaft to come back out. Therefore, having a shock cap with a vented hole in it, above the bladder, will simply use the spring of the rubber bladder to affect rebound.

Bladder with a Non-Vented Cap
Now consider what happens when you place a bladder in between the oil and the space inside a non-vented cap. This space will contain air; which is compressible. As you push the shock shaft in, displacing the oil, the bladder will push into the air chamber behind it, compressing the air as it moves inward. Now when you release the shock shaft, the compressed air behind the cap will assist in pushing the bladder back out. The air now acts as a small spring behind the cap. Higher rebound is easily achieved when building with bladders and non-vented caps.

Bladder with a Vented, Sealable Cap (bleeder screw provided)
Here's where things can get interesting. The volume behind the shock cap has air in it. With the shock shaft fully extended and no pressure in the shock body from the oil, the bladder is at rest. The air behind the bladder is neither pressurized nor has a vacuum on it. Leave the bleeder screw out of the cap, and again, you get no help from the air pressure as the air escapes from the bleeder hole in the cap. Put the bleeder screw in the cap, closing the cap, and you now get added air pressure behind the cap as you compress the shock shaft.

But, what if you push the shock shaft in halfway with the bleeder screw out? The bladder will deform slightly, due to the displacement of the shock shaft in the oil. It will also NOT be pressurized because the air can escape through the bleeder hole. What happens now if while holding the shock shaft at this position (half stroke), we then insert the bleeder screw, closing off the air chamber in the cap? As we let go of the shock shaft, will it push out or stay still (no rebound)? If we were able to create enough vacuum behind the cap to equalize the elasticity of the rubber bladder, in theory, the shock shaft would have no rebound from this point. It won't move. Push the shaft in slightly and we begin to pressurize the air behind the cap. Pull the shaft out slightly and we begin to create a vacuum behind the cap.

Now go to extremes. With the bleeder screw removed, push the shock shaft all the way in, deflecting the bladder completely, then install the bleeder screw. In this configuration, in theory, you begin to create a vacuum behind the cap the instant you start to pull the shaft out. This vacuum might then want to pull the shaft inward, creating the opposite effect of rebound.

Watch Gord Tessman in this video as he discusses this when building the new HB shocks.

http://www.dailymotion.com/video/x3015ls

Hybrid - Vented Cap, Bladder with Foam Spring
Another hybrid form of achieving consistent rebound, as mentioned in a few other posts, is the use of a vented cap, with a small piece of foam rubber behind the bladder (in the air chamber). The theory behind this was to maintain consistent rebound throughout the course of a race (when facing 30 to 45 minute nitro mains). The theory is aimed at a shock's temperature changing due to the friction of the parts (making the shock warmer) or cooler ambient air (making the shock cooler). Either way, air temperature affects air pressure. Therefore, a shock with a bladder and a closed cap (no vent hole) might have different pressure behind the cap, should the shock temperatures changes during a race.

So, pick your poison. Personally, I build bladder shocks with vented caps. I drill an additional 1.5mm hole in the cap to ensure the hole is large enough to let the air volume breathe in and out, with no restrictions. I am able to build a fairly neutral shock (very little rebound or pull back), relying on the bladder to be the only push/pull spring, not the air pressure or vacuum. My shocks are fairly consistent even when I neglect them longer than I should. The only drawback to the bigger hole in the cap is that it sucks a little more dirt in behind the bladder. Does not appear to affect performance as this dirt never gets in the oil nor is enough to fill the cap.