I think with POCO we already have a great foundation to build an excellent GUI library on top of it. It's not so much a technical problem, but more a kind of manpower issue, as Alex noted. So, if we're able to find enough contributors for such a project, we can do it.
There are already a few GUI libraries out there worth taking inspiration from. For example, gtkmm, the C++ wrapper over GTK+ is quite decent. Then there's .NET and Java (Swing and SWT). I'd say, take the best ideas from those and POCO-ify them. Also, I think we should keep the UI layer as simple as possible and not invest too much time into sophisticated theme engines, etc. While it is somewhat funny to be able to switch the UI from Motif look to Windows look with a menu command, I have never seen any practical use for it... The main goal should be to look as "native" as possible on every platform. This means using the respective platform's native widgets instead of emulating them. For Windows, that means to bite into the sour apple and use the WIN32 api. On the Mac, we should go with Cocoa. And on Linux/Unix, I guess it will be GTK+.
To get started, I have created a Wiki page
for this topic. Keep your ideas flowing...