Just as in backpacking, there are many separate activitiesm that can be classified as bicycle touring, you need to define what your primary activity is...
3-day tours vs. trans-am or trans-world; camping vs. staying in b&b; etc...
I would not get a 'comfort bike' for bicycle touring. I would (and did) get a touring bike or a fast-touring or randonneuring bike.
If you are only going to do local or close-to-a-bike shop touring you are safe with any frame. But, if you plan to do any remote or third-world touring, you'd be much better off with a steel frame (cr-mo) bike. I personally prefer steel or titanium bikes anyway and would never purchase an aluminum bike except for a full-suspension mtb (IMHO).
For a touring bike, do not get the popular few-spoke wheels, get normal traditional 32 or 36 spoke wheels, although if you are looking at mainstream bike stores, they may be as tough to find as UL gear at REI.
You will need to have some way to attach your luggage (as light as it may be) to the bike, so you will need rack attachments unless you use a Carradice-style Transverse saddlebag. I would also look for front rack attachments.
Other things you may want to look at are: front light attachments, fender attachments and room for fenders, room for tires larger than 28-32mm, gearing low enough for climbing big hills with your load (i.e. a triple or compact double).
Finally, most importantly, you will probably not be racing in your local crit series or Le Tour on your new touring bike so make sure it fits. Generally bicycle store employees will be completely unfamiliar with touring bikes and will size you on a bike based on the way Lance Armstrong looks on his bike. You will want the largest bike you can fit on for comfort over the long haul.
Sources for information include: the bicycle touring list @ phred.org, adventure cycling, Rivendell bicycle works, harris cyclery, vintage bicycle quarterly, randonneuring usa, Peter White Cycles.
For UL touring, I would recommend a randonneuring bike (randonneuring is ultra-distance, timed, self-supported riding). These bikes are light, fast, comfortable, and designed to carry a load. They will be difficult to find. Sources include: Heron Bicycles, Rivendell Bicycles, Wallingford Bicycles (Berthoud). Other good touring bike companies are: Bruce Gordon, Rodriguez, Co-Motion, ANT bicycles, IF has a nice fast-tour/rando bike as well.
THE source for panniers is Arkel OD. They make the best panniers in the world.
I hope that's not an overload. If I were to make it simple I would get a Heron Randonneur.
Good luck and have fun!