Or you can just use Qt's Designer which makes it very fast to knock up a GUI. (and then later evolve it into the real thing as well)
Not bashing QT, but QT designer is not a wireframing/mockup tool.
The point of wireframing is that the client doesn't look at it and go "this detail is wrong!" Wireframes and mockups should only communicate pre-prototype ideas, work-in-progress ideas and workflows. I think you might not understand what a tool like this is for if you think QT creator is better than every sketching tool on the market. Not a personal attack, but we seem to be talking in circles already.
This is one of the reasons why find people taking pains in some of these type of tools to not make it look like the real deal, because the closer it looks to the real deal, the more some people get hung up on irrelevant details.
Although I would agree that they often (but, ime, not always) appreciate an export (eg pdf) that can be signed off and stored.
We've had some project managers who were almost the most enthusiastic team member about interactive prototypes because they were the most effective (and understandable) thing to demonstrate/give to customers.
(To your general point though, I entirely agree, I wouldn't use QT Designer for this role -- and I'm also quite a fan of pencil, because it feels intentionally plain/bland.)
I feel like I'm being a pedantic a-hole in this thread, but the terminology of UX/IxD and the process seems like one of the more misunderstood aspects of building software and as a lot of developers (in my experience) want to either dismiss the process as unnecessary (they don't need to be told what to do) or they want to take on the process themselves (experienced coder, of course I can analyse and plan a user interface). I'll stop now before I start ranting.