Why would it be so difficult to include pixels option? For people who want to export pdf's and not print physical stuff.
Again, it's not a matter of difficulty, it's a matter of the issue that the entire idiom of Pages (and any other word processor) is for a print document. Therefore, all documents must, by definition, have a "physical" size.
And pixels simply have no meaning unless you also specify a dpi setting, which just isn't a feature most word processors bother with, since the percentage of people using them to create physical documents (versus web pages) is EXTREMELY small.
Apple, after all, already makes iWeb for this.
And on the issue of PDF, it's the same thing--PDFs, at least as generated by OSX, are also measured in inches, not pixels. PDF is, after all, originally and primarily for making a consistent digital representation of a print document.
If you want this to make more sense, think about it this way: A PDF has no "natural" size on screen--it's as big as you want it to be. An image within a PDF can be ANY resolution, regardless of the surrounding text. For example, you could have an image that was 1000 pixels high and only was the same visible height as two lines of text. It works, because the image is scaled to the correct size on screen, regardless of its pixels. This is the same way that Pages (and Word) operates.
If you put an unscaled photo on a web page that is 1000 pixels high, it's either HUGE, or that's going to be some seriously large text.
The point I and others are trying to make isn't that it's technologically impossible to have Pages let you specify sizes in pixels, it's that Apple would have had to set a specific dpi assumption for the app (or, alternately, let you set it), and that's just not how any modern word processor is set up.
In fact, even the web is gradually moving away from this--as displays get progressively higher resolution, less pixel-rigid units are going to become more and more desirable (David Hyatt, Safari's main developer, has some blog entries on this). After all, if you've got an ultra-high-resolution display (say, 200dpi) and the browser actually renders "12 px" text as being 12 pixels high, that will be TINY--0.06", which is nearly unreadable.
We're not there yet, but if display tech improves at the current rate we will be (and Apple is already most of the way to a resolution-independent MacOS interface for the same reason--the tech is there in Leopard).