In the MIX10 Event held recently, Joe Belfiore, Corporate Vice President, Windows Phone at Microsoft in his session- Changing our Game – an Introduction to Windows Phone 7 Series showed 3 Windows Phone 7 Series devices during the session.
At present these are the only 3 WP7S prototype devices. They are from Asus which was also used in his session for the demo, next is from LG which also has a slide-out keyboard and from Samsung. Here are some screenshots and the excerpt video taken from that session which shows these 3 WP7S devices.
WP7S device from LG
Check out this small video excerpt taken from that session which shows these 3 devices -