Equipment
The right bike will make all the difference to a tour. Nothing is worse than riding for hours each day on something uncomfortable which is creaking and feels inefficient. I don't own a touring bike and have therefore only toured on a mountain bike and, once, on my road bike for a lightweight trip to Northern France. If you don't mountain bike, a touring bike will be the most efficient means of carrying gear on everything from tarmac down to smooth tracks. You do not want to be riding skinny tyres and drop handlebars on anything much rougher or muddier though. Leave that to the cyclocrossers.
A road bike is great for 'credit card' touring in the developed world but not much good when you want to carry more than a backpack.
Frame
It is very important that this is strong and fits well. After that, I think that it is up to you. My mountain bikes have been steel, which is strong, easily welded and comfortable day after day. However I don't think the difficulty in welding aluminium should put you off. A strong aluminium mountain bike frame is highly unlikely to break. The ride will be 'harsher' than steel though.
Specialized make some excellent frames at a good price and buying a complete bike from them will reduce costs further still. I had a '97 Marin Pine Mountain until I discovered a crack in the frame last year.
As a replacement, I invested in an Independent Fabrication Deluxe. Very expensive but they make everything fully custom, sizing, tube selection, bottle mounts, cable routing, rack mounts, the lot. Give them a call as they are very happy to talk things through with you. The Deluxe is their classic mountain bike frame and rides beautifully when used for its intended purpose. It was pretty good for touring on too, fitting perfectly. Alternatively they can build you their Independence touring frame to your exact specifications. All for a price of course!
Alternatively consider the classic Koga Worldtraveller which is the 'sensible' touring bike of choice for many of the European riders that I meet. By all accounts they are bombproof and very comfortable, coming with all the racks, bottles and mudguards required for round the world excursions. I suspect they'd be just as at home on shorter trips and probably work well for commuting too.
Wheels
Possibly even more important than the frame, particularly on a rough tour. They will be carrying much more load than normal and since a loaded bike is less manouverable, you're more likely to hit things hard. I rode all my tours up until South America with Mavic 717 rims, a standard cross country mountain bike wheelset. Whilst they are fine for cross country and for touring mainly on the road, they weren't really up to touring off road. On the Great Divide I broke two of these rims on the back wheel.
Before going to South America, I had a set of Sun Rhyno rims laced to 36 hole Shimano XT hubs by St John Street Cycles. They have proved utterly indestructible and are still as round as when I left. I've been very impressed with this but they are very very heavy. This wheelset would be overkill for use where spares were readily available.
I used the excellent Schwable Marathon XR tyres for the South America trip and, like the wheels they were bombproof but very heavy and rolled rather slowly. I could have got away with one set for 5000 miles though and I got only one puncture in that time. Not bad going.
Components
The best you can afford. Remember that the latest thing won't have filtered out to the third world yet so the best bet is to avoid things like integrated shifters/brake levers, outboard bottom brackets and hydraulic discs in the back of beyond; they are overkill for touring anyway. Shimano XT is a good solid benchmark for components and I've never had a problem. Expect to replace a chain every 1000 to 2000 miles depending on how hard you're riding, but do it early to avoid having to replace the cassette as well (a worn chain will wear the cassette which will then slip with a new chain). This really happens and is not just a bike-shop ploy to get you to buy more stuff!
Rack or Yak?
I have used the BOB Yak trailer extensively and find it to be excellent. I enjoy riding off-road and have no doubt that the Yak is superior in this application. Its single-wheeled design tracks brilliantly, is much narrower than panniers, takes weight off the bike, and can be easily detached to give you your mountain bike back again. Another definite plus is the fact that you only have one piece of baggage to get into hotels or onto aeroplanes. This process is significantly faster than faffing with 4 or 5 separate bags, you simply pull out two pins and wheel it away (the top tip here is to do this whilst sitting on the back wheel of your bike in order to stabilize it - otherwise you run out of hands).
Of course, there are some disadvantages too. The trailer is heavier than racks for a start, at 7kg empty. I've also broken mine on three seperate occasions (without exceeding the weight limit), though it has always been easy to weld and BOB replaced it for free and without fuss when I was back in civilisation. Racks break as well, but I do think the Yak design ought to be stronger than it is.
It is also worth investigating the BOB Ibex suspension trailer which takes some of the strain off the frame over bumpy ground.
Note also that you can fit North Face or Ortleib 90L bags in the trailer. Having tried the standard BOB version (fully waterproof but not as large or strong) and the North Face (very convenient with its zip, and extremely robust, but not fully waterproof), I'd be inclined to suggest the Ortleib version which should provide the best of both worlds...
Old Man Mountain make some innovative racks that are well built and fit to bikes without rack mounts (mounting to the brake mounts and skewer for example). They'll also work on full-suspension bikes with disc brakes.
