I was wondering if this would increase the risk of dust and water entering the camera or is it designed to fit so tightly that it seals? I would have thought that they would put an O-ring on it but maybe I'm just being pedantic?
I think it does negatively impact the weather/dust sealing of the body. The design is consistent across all the 'sealed' models - even the 7D (which has a higher level of sealing than the 60D) has no seal for the grip.
In fact, they wouldn't have to have an O-ring all the way around (nor would they - O-ring seals are few and far between outside of the 1-series). The non-gripped body is sealed at the battery door by a foam seal that is on the door itself. It would seem to be very simple to design it such that the seal is on the body where the door connects, and thus a similar surface design on the grip connection would provide equivalent sealing at the battery compartment opening.
As it is, the 'seal' with the grip attached is only gravity - water flows down. If you use the body with the grip attached, in the rain, and then invert your camera, it seems to me that water that got between body and grip (and the curve would seem to direct it there) will flow 'up' along the battery compartment, reaching the contacts where you obviously don't want water.
This is one reason I prefer a 1-series body for shooting in the rain.