I doubt that Apple could really do anything but complain if another company used an Aqua-like UI.
You can't copyright something like "Aqua" because it's a style, not an actual collection of elements.
The only thing preventing other companies from copying it, is they would in turn be promoting Apple's look. It also isn't very original anymore, as Uboa mentioned.
Now using an actual tool of Apple's to create a UI, might be on the edge of what would be considered okay. Again, if it came down to it Apple might have a tough time proving that you did indeed use their tool to create the graphics in question.
A simple, I used Photoshop answer could get you out of that one. But then again, would you even want it to come down to that? It sounds to me based on the fact that you posed this question, youre having slight feelings of doubt for a possible misuse of the app. And so you should, its original purpose is for software development, not web page development.
So my advice, do what you feel comfortable doing, but continue to look into wether or not this is indeed an acceptable use of the app. My first guess and instinct would be to say no, but I dont have the developer tools installed, so I cannot be certain that this would be the correct answer.
