In the past, I wrote a server app that was supposed to be cross-platform (Windows, Linux, Embedded Linux and eventually OSX). I chose to write it in C++ & Qt. That ended up being a bad decision because of the bloat. Since we needed the app to be able to run on embedded systems we couldn't use Qt. (We eventually wanted to be able to run on VxWorks as well). So, the size of the binary was too large. Eventually, I rewrote the app in ANSI C and saved a huge amount in the size of the static binary IIRC ~4MB (Warning, I don't remember exactly). The #ifdef code for each platform was one .c file for each OS with just a few functions on how to start/stop the service etc...
I should take another look at it.
But in general it's pretty good. There was a GSOC project this year to implement a DirectX backend to the Windows drawing system instead of GDI but I don't know how well that has got on. They overhauled the drawing system to using native drawing contexts on whichever platform you're running, so you can do aliasing etc.
It's worth a look I think, as it is quite easy to write something usable in it very quickly. There is no database support anymore though - wxODBC died a death.