Not only are all current GBA emulators flirting with copyright laws, they're now violating a Nintendo patent.
Nintendo has recently been awarded a patent for "a software emulator...emulating a handheld video game platform," or in English, any and all Game Boy emulators. Those listed in the patent give the examples of cell phones, PDAs and the video screens found in airplane seats, but in effect, the patent refers to just about anything that duplicates or enhances the Game Boy line's hardware via emulation.
Now that Nintendo is the sole, legal owner of GBA emulation, any and all emulators are now just as illegal as the ROMs (though some people will argue otherwise). Companies such as Crimson Fire, which released a GBA emulator for the Tapwave Zodiac PDA last month, have recieved a cease and desist order from Nintendo, citing their patent as grounds for the suit.