I'll be writing up a review tonight since I've just managed to finally give the PS2 some downtime (my wife and I spent our free time finishing Final Fantasy X).
UPDATE: here's the promised review.
Among the latest crop of products available from the lik-sang.com website (famed exporter of desireable games and accesories not available in the U.S.) is the "Hori Controller" for Nintendo GameCube. This item might not have been nearly so desirable before the release of the GameBoy Player in the U.S., because from it's design it was clearly intended to cash in on it's release in Japan. Thankfully, like GBA games themselves, controllers are in this case ubiquitous - what works on the Cube there works on the Cube here.
How does this exploit the GameBoy Player, you ask? Quite simply by adopting the very "nostalgia" feel that many GameBoy Advance games have in the first place. It's been well known for a while now that some of the Advance's best titles are for the most part updated ports of their Super Nintendo equivalents; while even new games such as Castlevania Aria of Sorrow and Metroid Fusion look like redefined and improved sequels to their SNES counterparts, and often play that way as well. Quite wisely, the Hori Controller recognizes that and attempts to emulate an SNES controller in the "dogbone" style. With a D-pad on the left, Cube style A and B buttons on the right, start and select buttons in the middle, L and R shoulder buttons on top, the package is about 90% complete. A perfect copy would have had A B X Y diagonal buttons, but then reprogramming the X and Y could have been complicated, and with the design of most GBA games is really unnecessary anyway - Super Mario Advance 2 and 3 co-opted their functions to other controls regardless.
Several features take this controller beyond just a cheap knockoff of the SNES era though. First, the surface underneath is ridged, not flat all the way across, and the ridges actually form natural grips for your hands by being a half inch higher along the left and right hand sides. Secondly, and this is important to note for those who have been using the Cube's link cable with an Advance for a psuedo-SNES controller already, the Hori Controller has a "Z" button which the Advance lacks. Therefore, you can control the extra functions of the Player, as well as access the menu to swap games in and out; whereas before you had to keep a Cube controller in port one just for the "Z" shoulder when you needed it. Eliminating clutter always gets a

from MMF. Third, it's much more natural to play the GBP with this controller than a Cube controller itself, because the unneeded analog sticks are not present and don't get in your way. Again, another

.
Now there were a few drawbacks, but they seem relatively minor overall. The D-pad was a little "stiff" right out of the box, requiring a harder push for directional control than I'm used to. I'm sure in time it will loosen up a bit with repeated use. The shoulder buttons tended to "click" a bit, which went unnoticed in time but may annoy some other players. I only played for a couple of hours, but I did wonder if the "ridges" on the bottom might eventually become uncomfortable, especially for those with smaller hands. It does make the controller easier to grip, but if you're already used to having a firm grip on an SNES controller to begin with and treat it the same way (not hard to do given it's the same style) you may take even more issue. As for gameplay itself, I tried both Namco Museum (remarkable how good the graphics look blown up with the Player) and Metroid Fusion (beautiful as always) and found them no more or less frustrating to play than I had with the Advance alone. In fact Galaga and Galaxian were much easier to play this way, or than with the player using an Advance on link cable. I have to get used to the Hori Controller's larger shoulder buttons after the small ones of the GBA and the SP, but in time I'm sure Fusion will be as smooth if not smoother.
Bottom line? If you intend to use your Game Boy Player a lot, the Hori Controller is the accessory to complete your perfect set-up. However it does come at a price - 19.99 US + 7 to ship it to the States. It's possible that if the Player takes off here, a similar (and/or cheaper) controller may be available here. I decided not to wait though, and in my case I feel it was worth it. Keep in mind though that if you intend to use it as a Cube controller, don't - without the analog controls it will be worthless for them. Overall rating? 8.5 out of 10.