The Pre Oreo effect—as in, twisting an Oreo—is definitely an unwanted and unpleasant flaw some have experienced in the Pre's hardware. Pre Central has a few DIY, probable-warranty-voiding methods to fix the problem.
Though Palm or Sprint will likely replace a Pre with obvious wiggle, sometimes you just want to get your hands dirty and do it yourself. The problem seems to be a bit too much give in the tabs that hold the two halves of the phone to the sliding mechanism, so you can fix it by bending the tabs (risky) or even applying a layer of superglue to the rails to make them thick enough to fit the too-loose tabs (riskier). It'd take a confident tinkerer to mess with such delicate hardware, but anything to avoid dealing with wireless carrier customer service, right? [Pre Central, image from same source]