[anti]

nice shots, i like the crop on the girly girl; sky and rock shots are well done too happy:
the 18-200 is a good lens, my brother has the sigma 18-200 and it's pretty much all/you'll need for cheap.

in terms of your autofocusing problem, try this. it works in both manual and auto modes. go to menu and go to the pencil on the left tab.
you'll see
01. beep (on/off)
02. autofocus (AF-S/AF-C)
03. af-area mode (single area/dynamic area/closest subject)
04. af assist
use the directional pad to flip through and change settings. in 02. autofocus there's two settings AF-S (single focus) and AF-C (continuous). single focus will only focus in on once and once it does find it's focus, it stays with it. if the camera can't focus it wont let you shoot the shot. continuous is just simply holding down the shutter half way and the autofocus will continuously hunt for the focusing point. i find this is best for a few reasons. one it's continuous, whatever you keep it on will be in focus to the best of the camera's ability and if you decide to focus on a subject and move it yet you want to lock the focus on that subject just hold down the ae-l/af-l button; it will lock the focus on whatever subject you first focused on.
secondly 03. af-area mode. i find dynamic area is the best as it allows you to use the directional pad and select the area you want the camera to focus on. closest subject is just inane i feel, and single area doesn't have the freedom of dynamic area. hope this helps your autofocus problem.