Camelback vs. waterbottle

Bottles give you more flexibility for quick and easy refills, especially during a rcae where bottles are what they hand out. A camleback might be a little too much extra gear for a supported road triathlon.

I think aerodynamics have very little to do with it, since Camelbaks can be pretty aero, depending on the model. The main reason most triathletes don't use them is that you can't refill them easily. Most triathlons are either short enough to do with just a couple of bottles or have aid stations along the way. There just isn't the need to carry the volume a Camelbak holds during a race. That said, I always use a Camelbak on my 50+ mile training rides because I find it more convenient in that situation.

I also come from a mountain bike background and have used both CB's and WB's. I cannot get comfortable with a CB on when in the aero position so I stick with water bottles when on my tri bike.

i use a camelback and am most comfortable with that.  i am petite and have a small bike frame.  average size water bottles sometimes get "stuck" and i can't pull them out while riding.  with the camelback, i have constant access to water anytime i need it.
i have only done sprint tris, so i don't know what i would do on longer distances. 

Since I do a lot of both, I love to train with my CB since I live in the desert.  I will use it for any ride over a couple hours and for ALL of my MTB rides.  I also ride in very rural areas, where the oportunity to refill is rare.

For racing, however, I only use bottles.  I have an aero bottle and seat mounted carrier.  For the longer races, they have aid stations about every 10 miles and they give out bottles.  You don't have to carry as much (1-2 bottles) which keeps the weight down AND it's easy to swap bottles.

When on the road with my mt bike, I always use bottles.  I would prefer having the weight carried by the bike rather than my back.  I find that when I'm hunched over for extended periods of time, I'm more prone to back discomfort when riding with a pack of any type (I used to commute to/from work carrying a laptop, which was even worse).  I usually ride with three bottle cages which is enough for 60+ oz of water (or 40 oz of water and a battery).  Usually more than enough to get from one park or water fountain to the next.

On the trail, I usually ride with a camelback, although it's more because of the need for additional gear than because of the ease of access to water.

I believe that the camelback, especially when it's under your jersey is extremely aero.

The problems are refilling it, which a number of people have said already.

But I suspect it's also a matter of comfort.  When you're upright on a mountain bike it might feel fine, but do you really want the weight pressing down on your back when you're tilted forward in the aero position?

Plus there's ventilation.  Anything on your back is going to trap heat and sweat, rather than letting it evaporate off.