I've done 3 half IM's, all wearing tri-shorts.

For the first one I had a terry butterfly saddle and a road bike. By about 25 miles I was in agony. I spent the rest of the bike portion moving around constantly to try to get my butt comfortable.

For the second one I had the same bike but switched the saddle to a terry liberator. This was a lot better- it's a little bit wider and I was okay on it with the tri-shorts.

For the third one I had a tri-bike with a terry liberator saddle. I had no butt problems whatsoever. (I think the aero position takes some of the pressure off the butt, I was a little worried about the bones in the front but they were fine.)

So I think it comes down to how well your saddle works for you- if it's a reasonably good fit, you'll probably do fine in tri-shorts- if your saddle isn't quite right, it might be torture. Try a long ride or a shorter tri to test it out first (I had done an Olympic distance race before the first tri and noticed my butt was starting to bother me towards the end- I hadn't realized how bad it would be over 56 miles.)

Good luck!