[Cainehill]

Not that difficult - in the 90s I wrote GUI and graphics libraries that worked on SunOS/Solaris (Unix) and Windows both, using OLIT or Motif on Unix and MFC on Windows. Admittedly the stock GUI facilities aren't what you want for a game, but it was perfectly suitable for wargaming - plain buttons & menus and whatnot, but scalable map images, terrain and unit icon (with barometer) overlays.

And these days, there's at least two widely available GUI libraries that work on at least Linux and Windows, possibly macs and others (GtK and Trolltech's QT). Then, if you want hand-drawn _graphic_ interfaces, there are multiplatform graphic libraries - the one Illwinter uses, I'm not sure if Demon Whiskey (Bard's Tale clone) used a different one. But then you need a graphic artist _and_ someone who can code a custom GUI with pretty graphics.

Still - as long as you're willing to limit yourself to Linux, Windows, and possibly Mac and commercial Unix's, it's definately doable, and for lowest common denominator UIs, easy.