At this point in time the Web based Slingplayer is unable to be switched to "video only" mode. Just as a suggestion, you could re-size the window your using to view your Slingbox in and move that around just like the pop-out window from our Windows based Slingplayer.
Hope this helps,
The Slingbox Moderation Team
I hope that "at this point in time" means that you will be adding back this facility in the future! Because of its absence, I'm reinstalling the "legacy" version of the player.
I don't really understand the suggested workaround - the pop-out player is a feature of the web player not the old one, and resizing the window won't get rid of the window frame and address bar