Also, Postscript used to be pixel fonts, which means letters are saved as pixel maps, like a bitmap. So once you scale a font, you better hope a pixel map for the size you want exists, else the text tool has to calculate the new size which will look chunky because pixel "stairs" will appear, no matter how good any anti-aliasing is (which again make them blurry). TrueType saves the fonts as outlines, which means they only save a mathematical function to describe the fonts outlines. Once the font is scaled, all the text tool has to do is change the parameters of the function and redraw the "new" letter. So you can get ANY size you want in exact the same quality. The only problem then arises when it is sent to a RIP, where it has to be rasterized. Expensive fonts have a good hinting here so serifs or stuff like that stay sharp once the text is translated into a pixel map (which the printer needs since it can only print in pixels (or small dots)).
The fonts in MacOS X are definitely outline fonts, allthough you still can use Postscript fonts of course. Allthough you should be cauteous when installing the same font as Truetype and Postscript, since the onscreen display will be truetype but the printer will most likely use the postscript file, but I think you know that.