My guess is that the 75u is targeted at fighting Vizio and other low-end plasmas from taking sales (it is already $2000 at BB--and I think BB only sells at MSRP). The 77u is their 'upsell model.'
I saw the 77u compared to standard plasmas at Magnolia with a bright window in the background. The difference was pretty pronounced to me. With the display off, it looked like a blurry reflection (but just as bright) so I wasn't too impressed. Once I turned the display back on, however, it became clear that the anti-glare is working. Because I couldn't see the sharp reflection, my eyes tended not to focus on the reflection of the outside. If I paid attention, I could tell the light source of the reflection was still present, but it was far less distracting.
My major complaint is that they didn't include the anti-glare on the 700/750. It is honestly enough of a feature (I have a bright sliding glass door in the room) to make me choose the 77u over the 700.