Well you could use the usual Photoshop, illustrator .... But also check PaintCode it's a mac only app that helps create vector based UI (in objective-c) for iOs only.

Love to have something similar for Android (no sencha) 'cause the UI for android can be a serious pain.

Basically PaintCode can import svg and photoshop layers and convert them into objective c code. Which can be easily reused, and reconfigured for any new resolution that may come out.

It's not perfect but it's pretty neat http://www.paintcodeapp.com/