I'm still undecided. Did some research on the size differences between the cases I'm considering and thought I'd post the results. All the measurements other than the Otterbox were taken by forum member PGR (I 'borrowed' them from one of his threads) so kudos to him for perhaps making our decision a bit easier!
Bare phone: 4.465 x 2.225 x 0.556 inches
Body Glove: 4.643 x 2.405 x 0.804 inches
Innocase: 4.56 x 2.328 x 0.640 inches
Otterbox: 4.62 x 2.41 x .76 in. inches
Looks like the Innocase has the least bulk but what about protection?
The Body Glove looks to be the bulkiest overall.
Otterbox falls somewhere in between the two others in general. It's barely a bit wider that the Body Glove but that's probably including the cool rubberized volume controls cover. Plus I like that there are covers for the ports/jacks so crap doesn't get into them when not in use.
Edit: Should also mention that the Otterbox offers additional protection around the earpiece and covers the HTC and Sprint logos if that matters to anyone.
Could be interesting how the sound quality might be affected as a result. It's creating a nice little chamber of sorts for the earpiece that may cause the sound to be a bit more directional and also keeping it a bit further from the ear. I'm being entirely too anal about this...