After seeing the post for the Slingshot earlier today I have pointed out that you might find it is too small very quickly (I did).
This is a good alternative pack (or additional), bit bigger yes, but much more versatile which is why I upgraded to it. Found it for a really good price on Amazon at the moment so thought I would post the deal.
"The compact, lightweight design of the Flipside 300 backpack lets you carry your pro digital SLR without worry. The unique back compartment entry gives you safe, easy access to camera gear when you re setting up plus extra security when you re on the move "