An emulator with ROMS won't be liegal, period, since ROMS are essentially pirated, by definition (assuming you don't own the original cartridges). It's a more interesting question if we're talking about an arcade machine housing a console (like an Xbox 360, e.g.), kind of like what they'll have at Best Buy or Gamestop. With a console, most likely you'd be using legally purchased games, so the only question is whether there is a licensing agreement attached to the game that prevents you using it in a commercial setting, and that would require reading the fine print. My hunch is that it is outlawed in the EULA (as most things are), but that it's no big deal to do it. I mean, some bars will have "Halo 3 Night," or something like that, where they'll set up a console and projector and host a tournament. Having a for-pay arcade machine in a small establishment, containing a console playing legally purchased games, is a much smaller deal than that, and doesn't seem like it would pose a problem.