wxWidgets is another choice. Wouldn't it be easier to convert a library that already supports all platforms?
Been discussed. Issue was size, mainly because wxWidgets, even as seperate libraries, tends to go up to about 50mb with debugging info and without it's still pretty damn big. Not to mention it makes heavy use of overloading, so it's not the prettiest thing to consider wrapping. If you wanted to, you could go download wx.NET and use its C wrapper to get a head-start on it though. I considered it, but it's not an easy thing to do no matter how you look at it.