Judging by what little information is on the FPGA Arcade site, I'm guessing it'll be something more like Minimig where it's a reimplementation of an A500 - NatAmi is more geared towards people who use or want to use a 68k Amiga for serious day-to-day computing. Not that there isn't overlap between the markets, but I'd think one hardly kills the prospects for the other.
I'm not going to get into Natami bashing here, it's a great project driven by some talented engineers.
I accept the comment about the lack of info on the FPGAArcade web site. We are currently developing a new web site. We have been busy producing the board and working on the firmware. I'm in china at the moment and have just send the 3rd revision data to manufacture.
/MikeJ