You can develop Qt apps with ease on Windows with Eclipse - all you need is the QT IDE/SDK, QT Eclipse Integration plugin, and eventually MinGW if you don't install it with QT IDE.

GTK development on Windows is a bit trickier (and may require a lot of porting work when moving it to Maemo), but that topic has been discused all over the interwebs, so if GTK is your flavor fire up Google.

When you're done, all you need is to recompile your app against ARM using Maemo SDK (at which point you'll see a huge advantage of Qt over GTK development), and there are several ways to do it. Probably the less painful (no VMs, scratchbox etc.) is the MADDE approach, although it currently lacks a N900 emulator so you'll need to test your apps directly on the device.