Close enough. Proof - drop a playing card on your foot - now, a bowling ball.
The momentum - mass * velocity - is what hits at the end, and it has to be transferred somewhere - usually into the device, case, or cover.
Sometimes a lighter device has less momentum, but less protection from momentum transferred back - but in general, if light enough, it's not an issue.
I think the new Casio G'z One Commando is pretty cool btw - except for being on Verizon and having Bing slathered on it - for a ruggedized phone it shows a lot of features to my eye, especially for outdoor use. I like the as-built G'z Gear configurations for it (G'z is pronounced Jeez, as in Jeez, what a dopey name for their stuff.)
http://www.casiogzone.com/commando/
Discussions for it here -
http://androidforums.com/casio-g-zone-commando/278977-casio-c771-gzone-commando.html
I don't care about weight because I almost exclusively use a wired headset.
I can't stand holding anything in one position for too long. I know, I'll do it for you, dan -
EarlyMon - The GirlieMon! lma0