Qt for X11 was GPL since at least Qt 3, which certainly means you could legally link it with other GPL sources in any manner. I believe Qt 4 went LGPL for all platforms, so now I believe you can link it with pretty much anything that's open source, and dynamically link proprietary applications.

I am not a lawyer and some people disagree with how to interpret the rules of linking, but I think this is mostly the conventional wisdom.