1) My browser is not maximized. The size of my screen doesn't matter. The size of my browser window does.
2) The iPhone's screen resolution is 480x320. NewEgg currently lists at least one 1920x1080 monitor for under $200. Designing to either of those resolutions will make your site completely unusable on the other. Even if you split the difference and design to 1024x768, you'll get a stripe covering half the screen width on the $200 monitor (which, IMO, looks like crap) and it will still be completely unusable on the iPhone.
Screens aren't just getting bigger. They're also getting smaller. The trend is moving to fluid layout instead of fixed-width and it's for a damn good reason.
