Nokia has published a demo application for N900, available in extras-devel repository, that shows a preview of Maemo 6 (Harmattan) user interface. Here there are some screenshots of the demo:

Demo application main window

Question dialog

Text entry dialog

Progress indicator

Information banner

Event banner

Please note that installing this demo will also install Qt 4.6.2 on the N900 and about 52 Mb are required.