How can I unlock an iPod touch which has VoiceOver enabled?
The main UX change you need to be aware of with VoiceOver enabled is that a single tap selects an element and a double-tap anywhere on the screen will cause a simulated tap on the element which is currently selected. This means that you need to tap once on what you want to select, then double-tap anywhere.

Select Slide to Unlock, then double-tap. Enter your passcode by tapping to select buttons and double-tapping to confirm. Continue with this through Settings to disable VoiceOver.