what speakers do the best "disappearing act"??

I'm surprised no one has mentioned any ESL (except for the Quads--which do disappear). I find most of the ESLs to do a great disappearing act, when set up properly--and as Sean mentioned used with appropriate equipment and cables. Martin Logan and Soundlab speakers do wonderful disapearing acts.

Aerial 7B's and 6's disappeared nicely as have Proac 1.5's. Excellent smaller floorstanders. My favorite is the Eidelon, especially considering the size difference. Iterations from above; placement, associated electronics and cabling may dictate your success.

Abstract7, no one has mentioned ESLs because although they can and do disappear within a limited sweet spot, they tend to have non-uniform sonic dispersion in other parts of the listening room.

In addition to my ESLs I own a pair of B&W CDM 9NT speakers, which sound great in the sweet spot -- but much more uniform throughout the room.

My Ohm Walsh 300's do a great job in this category.

Many quality small monitors will do that "out of box" thing, Totems come to mind. I have heard very cheap Mission 771s do it. I think beyond driver and x-over design that baffle area and cabinet design have much to do with it. But on the other hand I have heard it from much larger speakers too. Adding a sub with a steep x-over and a quiet cabinet will help with that all around sound.
Sean was also dead on when he talked about a preamp making the big change in his system. The electronics you use will define the limits your speakers will achieve, and many believe the hardest part to get right is the preamp. Maybe Sean will mention the preamp that sounds so good with his large speakers?