Xcode is an IDE. It's not a 'multi-platform development kit.' It is not an alternative to something like Qt. But, as Trolltech's website notes, Xcode is a supported IDE for Qt/Mac development.
Adobe put themselves in a tight spot by not recognizing (or at least not reacting to) the handwriting on the wall regarding Metrowerks. Adobe's dependence on Metrowerks' IDE and libraries is similar to your story about the proprietary middleware.
I don't think Xcode represents the same risk. First, if you're targeting Mac OS X then you're already depending on Apple. If Apple loses interest in computers and becomes a consumer electronics company your investment in Mac OS X development may be a lost regardless of your choice to use Xcode or not. Second, Xcode has little or no direct competition right now. Paradoxically that's good. Apple needs developers so Apple needs to support good developer tools.
You are always "putting your coding effort in the hands of other people." Even when you are using open source. The potential advantage of open source is that by providing more options the risk factors are changed.