Taking the iPad2 for a spin! SketchBook Pro app ($4.99) + Pogo Sketch ($14.95)
Verdict: Great for quick sketching & using basic colors. Small learning curve for navigating the UI via gestures, but once you get a handle on them the workflow feels fairly fast and efficient.
For fully customized color palette & detailed line art, bring it back to the desktop. The app exports to PSD so you can continue to edit it in Photoshop or Sketchbook Express/Pro for Desktop.