bike shorts under wetsuit during IM swim?

I use DeSoto Tri Shorts under the Wetsuit. 

I would experiment some in your training swims.  Compare using Bike Shorts vs Tri Shorts and see if their is any more binding, excess water soppage,  Hop on the bike and see how much water squishes out....

I wouldn't.  Bike shorts have a much larger chamois pad than tri shorts.  If you like the comfort of bike shorts, I'd just change.  But if you do wear them, just make sure you've tried it out beforehand.  Don't assume that it won't be a problem. 

Personally, I'll wear tri shorts for the whole race.  But I rarely wear bike shorts even just for biking.  I prefer the smaller pads.

Ever seen someone from behind running with wet bike shorts?  There aren't that many things I can think of that look more uncomfortable.  R-A-S-H for sure, I would think

Another option is getting a DeSoto gel seat cover and using tri shorts.

Wearing bike shorts and wearing them AFTER the swim are two different animals.  Definetly put them in the sink and jump on the trainer.  And for a LONG ride, not an easy hour ride.

Transitions in IM are not that super speedy.  Take the 20 seconds to change into them.  Then you won't have the issue/question at all.

I'm with Marv.  It's a long day, and you get the luxury of a changing tent.  Unless you're trying to qualify for Kona in your AG, I'd take the extra minute to put on dry stuff.  What if it happens to be cold like at IM Wisconsin last year?  You'll be freezing at the start of the bike loop.  The comfort factor will be enormous!

I would not suggest bike shorts being wet and then hopping on your bike for a bunch of hours.  Have you tried any long distance rides in tri-shorts?

I wore tri-shorts for Ironman under my wetsuit last year.  I did 4 or 5 160km rides in training in both tri and high end bib shorts with the most comfortable diaper-chamois I could find.

The tri-shorts were by far and away MUCH more comfortable over the long haul.  

If you're dead set on wearing a set of bike shorts for IMAZ, I would definitely change into them in the transition, baby powder or a tad of body glide on teh leg bands will make the transition that much quicker.  I think a big wet chamois starting the bike leg is a blister in the making.  Either route you take, make sure to use either body glide or lots of chamois butter on pad and you're golden.

b.