You would think that Nintendo could have implemented a secondary connection that emulates/cooperates with the GBA wireless adapter, but I guess they needed something to keep GBAs moving. I don't know all the technical nuances but basically, GBA wireless and DS wireless just aren't the same thing--they don't use the same frequencies, or codecs, I don't know.
I'll bet SP prices will drop during the holidays, and I actually plan on getting a backlit model if that is the case--and it's due in large part to Pokémon. Not only do I prefer the button layout of the SP over the DS Lite (as far as A and B are concerned), but evolving Kadabra into Alakazam is... a basic necessity. It might be worth picking them up if the price is right, and if you need an extra wireless adapter, Toys-R-Us has them on clearance for about $4 (though, somewhat strangely, the old game link cable is still $13).
...or, you can wait 'til an estimated Spring '07 for Diamond and Pearl.