Aren't you just sick and tired of hearing this question from your friends and family members and not having a decent answer? Well today is your lucky day.
Now be forewarned. This is one of those situations where more knowledge equals more complication. Before I reveal this mystery, let me be clear that manufacturers don't always clearly label when a ski has been manufactured for telemark or alpine touring performance. Furthermore you begin to notice the downright contradictions in most ski reviews. You may end up doing a lot of digging to determine whether a ski is "ideal" for a particular type of endeavor. You may be better off just not knowing. Read on at your peril.
Let's start with a quick review. The primary differences between telemark and alpine touring (or randonee) skiing are in the binding and boots. The telemark binding locks down the front of your foot, and has an adjustable cable on the back that is loosened for rolling terrain, and tightened to keep your heel down for descents. The telemark boot has a longer front edge, often called a duck bill. This leads to necessary adjustments on crampons, endless teasing by alpine skiers, and other inane stuff that Gered is more qualified to comment upon.
As for alpine touring, the boot is essentially a light version of the boot you use to go resort skiing. Additionally, the binding is also similar to the resort style bindings, but they will allow for the heel to be released to ski on flats, rolling terrain, or uphill.
Which brings us to the skis. What, if any, difference is there in a ski designed for alpine touring versus one designed for telemark skiing?
First, we start with the knowledge that most skis are cambered, even those solely designed for downhill resort skiing. Camber is the bow in the ski that causes the middle to rise off the snow when no weight is applied. Downhill skis have minimal camber so that they can easily hold an edge on packed powder. On the other end of the spectrum are nordic skis, which are designed primarily for touring over long distances on flat or rolling terrain. These have a higher camber (referred to as double camber) which reduces friction on the non-weighted ski, and allows for faster and more efficient travel.
Both telemark and alpine touring skis are designed try to get the best of both alpine and nordic worlds, and so they are both designed with camber so that they can travel easily on rolling terrain, as well as hold an edge on downhills. Therefore, the camber is usually the same for both.
The difference in the skis originates in the mechanics of a turn with each skiing style. Alpine style turning involves putting most of your body weight on the downhill ski as you turn, with only a small fraction of your weight on the uphill ski. In contrast, proper telemark form has the skier place weight evenly on both skis while turning. Therefore, a telemark ski must flex to get the full length of the edge onto the snow with much less pressure. Thus, the rigidity of the ski is the primary difference.
Now that you know the difference, good friggin luck finding the "ideal" ski. The typical ski advertisement will tell you that the ski is flexible but rigid... as well as tell you that the ski does well in powder and on crud, and goes fast while having a short turning radius. Note to ski manufacturers: I know you want to maximize your sales, but just freaking tell us what the ski does well. I'm sick of reading the same review for each ski. And believe me, I've read every one of them, because I'm a gear freak.
If you're alpine touring, you don't need to worry so much about the ski design, as a telemark specific ski will respond well to your weight, and get the edge down. As an alpine tourer, however, you are also free to engage a more rigid ski on hardpack which makes for a smoother ride. You may miss out on this added benefit if you stick to telemark specific skis.
For a telemarker getting the right ski is more important. A more rigid alpine ski may not make full contact with the snowpack with the lesser forces of a telemark turn.
So how do you tell what type of ski you're looking at? Start with whether the manufacturer has indicated it is specifically for telemarking. This may or may not help, as manufacturers don't like to box out potential customers (see above). However, if a ski is marketed as both a telemark and alpine touring ski it is more likely that the ski is flexible and therefore able to be used for both styles. Also pay attention to words like flex and torsional rigidity, particularly with skis that have a higher camber. The stiffer a cambered ski, the less likely it will perform well for a telemarker on crusty snow.
Now that you're familiar with ski vocabulary, I highly recommend using it in everyday conversations to impress and amaze your friends. For example, "Your reports have excellent torsional rigidity when I throw them in the trash." Or better yet, "Joe, you seem to be lacking your usual double camber today."
Okay maybe amaze and impress weren't the right terms.