^ too power intensive? seems to me that the only thing stopping gbc games from being played are the little pieces on the back corners of the slot(take a look at your gba cartridge, then look at your gbc cartridge)
and the fact that nintendo hates longevity of product... they always eliminate backwards compatibility on their newer stuff because they want you to forget about the good old times and buy the flashy new stuff... so much for loyalty...
There is a headphone adapter for the SP that plugs into the charger slot.
Oh, and Lloyd, Nintendo don't hate backward compatibility. The GBC was fully compatible with the GBC games. The GBA had almost complete compatibility with the GB / GBC (I believe there were 3 or 4 games that wouldn't work on the GBA, all localised to Japan, IIRC). The SNES and Gamecube also had add-ons that allowed you to play GB/GBC games (The Gamecube add-on was basically a GBA)
Don't forget that the first 2 DS releases (Classic and DS Lite) both had GBA support..
On the legal side of flashcards and emulation, don't believe the propaganda that ROM sites tell you, downloading ROMs is illegal (as is having the means to make a ROM image of a cartridge in most countries), and there was a European Union-wide law passed last year that outlawed buying & selling of flashcards. Those that were sold before the law came into effect are still legal, as long as the owner doesn't attempt to sell them on.
Where logic won't work - I will PAL Gamers unite - http://www.gamefaqs.com/boards/277-pal-gaming