12:42 p.m. on October 10, 2006 (EDT)
One thing that hasn't been explicitly stated here, though Tom alluded to it, is that there is solo and there is solo. For example, I hike in the parks and Open Space Reserves around here by myself all the time, several times a week. In some sense, these 10 to 30 mile, 2000 to 5000 ft total altitude gain "dayhikes" are solo. I often see no other hiker on the trail for most of the hike. In most of the parks there are large areas with no cellphone coverage (although I can hit ham radio repeaters virtually everywhere in them). But I usually let Barb know where I am headed, and in many of the parks, I know the rangers and chat with them at the start and end of the hike. These people know my approximate time for return. There are occasional exceptions, like the hike I did in Big Sur from the ocean to the top of a significant hill via a trail that is largely abandoned which took almost half again as long as intended (I got back to the car after sunset, despite having started just after sunrise). But knowing that it was an abandoned trail (still on the maps), there was no worry with the underestimate of time.
Much of the time on many of these hikes, I do pass other hikers (mostly solo themselves), sometimes a ranger on patrol, and (to my dismay) get passed on the way up the main hill and back down again by runners (one hike just before Bay to Breakers, I got passed by a couple of Kenyans, one of whom took 1st in Bay to Breakers that year, about half-way up the hill, then again at the 3/4 mark on their way back down, the trail being 5 miles each way and 2300 ft of gain).
The other meaning of solo is when you truly are alone, going into an area where the probability of encountering people is virtually zero. Examples of this are backcountry ski and snowshoe tours, backpacks cross-country that are mostly off trail, and some technical climbs in the Sierra, Cascades, and Rockies backcountry. Sometimes I intentionally do these in blizzard conditions in winter. Again, though, I have given an itinerary to Barb and/or rangers. True, like Aaron Ralston, if something happened, even knowing the itinerary, people would not know where to look necessarily. That's the time to play it extremely conservatively. Ralston was very experienced and generally cautious, but still got pinned by the rock. Timothy "Grizzly", on the other hand, pushed his luck way too far (and had a companion at that who also got eaten).
The general advice is to have the "Ten Essentials", which most people interpret as the "official" list in MFOTH. But blindly following a written checklist won't prepare you for what might really happen on your particular trip. You won't need huge amounts of water on a winter snow trip, but you will need some way to melt snow or know that there are running streams available (and how NOT to fall in). You probably won't need rain gear in the desert (clothes dry fast in those conditions), but you may need a lot of extra water. Matches won't help above timberline, unless you have a stove (which isn't listed as one of the "10").
So if you go solo, it is even more important than group trips to be sure you understand what resources are available, what resources you have to carry with you, what your personal limitations are, what risks there are and how to avoid/mitigate/recover from them by yourself with no help. Example - maybe you can avoid crossing streams altogether, but maybe you need to cross a fairly dangerous stream (how can you make it safer - no, don't tie a rope to a tree), and if the disaster happens (slip, knocked over by a current stronger than you expected, strainer coming up as you are swept downstream), how can you get out of the situation.
I suggest you read accounts by people like John Muir (an avid solo hiker, with lots of near misses), Ralston, Warren MacDonald's A Test of Will (a friend of mine, who was in fact with a companion - it was the subject of a Discovery Channel program), and see Grizzly Man, and read Accidents in North American Mountaineering (nominally about climbing, but includes hiking incidents as well). There is a book I can't put my hands on right now that has a title like "Survivors" or "Those who survive" about the attitudes and psychology of people who get into situations (often solo) and survive them.
No, I am not trying to scare people away from solo treks. After all, I do them all the time. As others have said (and you will get from Muir's books), some of the greatest rewards come from solo trips, things you do on your own, depending solely on your own wits and resources, knowing that you have prepared well. The point is - educate yourself as well as you can about what you need, what the risks are, and how to deal with them by yourself, with no help or aid from anyone else. Do not count on cell phones, GPS receivers, sat phones, emergency locators, or any other electronic technology (remember, I spent 20 years working on developing such technology, and I am delighted to have you buy the widgets - helps support me in my dotage). If you are solo, or even with a couple companions, help may be days away, even in the US. But that just adds to the joy of being self-sufficient.