The Gamepark32 is about as illegal as a laptop, PDA, etc.
However, Nintendo currently has a stranglehold over the handheld market.
In fact, this may sound crazy, but I think Gamepark would have better chances after the release of the PSP. Handhelds usually only live up to at most half of their hype. As people get rid of their GBAs for the PSP, those that regret their decision, may still want something a little more powerful then the GBA in their hands (as they've sold all their games). Of course, this is more of a select market, but in my mind it would be sizable enough to bring in a fair haul. The number who want the GP32 for emulation wouldn't change before or after the release of the PSP.