Gamespot is reporting that Electronic Arts mistakenly posted the following note in the news section of the NHL '07 product site;
"NHL 07 is right around the corner but why wait? The Xbox 360 playable demo is live on the 360 Marketplace now! Fire up your Xbox 360 and download the NHL 07 demo before it's available in stores." -Note on EA's NHL '07 page
Of course, the catch here is that the aforementioned Xbox 360 demo is NOT currently available in the Xbox Live Marketplace and is most likely trapped in quality assurance testing. While the note has since been taken down, it should serve as a good indication that at some point there will be a NHL '07 demo available via Live - hopefully prior to the game's release on September 14th.
For more information, check the referenced Gamespot article here and EA's NHL '07 product page here.