Shaun,
I'm not sure from the posts above whether you are still interested in a protection-trainable dog, or just want a big dog with an impressive look, a good disposition around the house, a long life-span, and enough stamina to go hiking with you.
If it is the latter, in addition to the Cane Corso suggested by Robert Van Camp, you may want to look into the Tibetan mastiff (or as it should probably be called, the Tibetan mountain dog). As a rare, primitive breed relatively new to the US and western-style "breeding" as opposed to the survival of the fittest on the high steppes of Tibet, they tend to have somewhat fewer genetic health issues, and they have a 12-15 year life span. My family has two (in addition to a Jack Russell terrier). TM's tend to be very difficult to deal with as puppies, lots of chewing on anything they can get hold of, but ours are now maturing nicely at almost 16 months -- ours are very distant cousins, not littermates, although they are only a day apart in age.
Adult males are 26-28 inches, 120+ pounds (some breeders really like the larger lines, which can go up to 150-180 in some cases); females are proportionately smaller, around 100 pounds. There are a lot of websites discussing the breed if you want further information.
You should be aware that they don't respond to the recall well as a general rule (hence the general advice "never off leash") and tend to have their own opinions, but that's part of the charm of the TM.
So far my experience validates the advice I was given that it is not a breed that would do well in protection training. Prey drive seems very low, but my male, at least, is very territorial and my family sleeps soundly knowing that
NO ONE will ever walk into our house unannounced, and whoever does so will leave quickly -- if they can. Outside, the TM has been described as a dog that will simply place himself between you and any perceived threat. I haven't seen a lot of that behavior yet, but maybe the sight of me walking the two of them discourages threatening behavior. <img src="/ubbthreads/images/graemlins/laugh.gif" alt="" /> At 16 months, mine are still not full-grown. It should be interesting when they are.
Although some say that they are hard to train in obedience, that hasn't really been my experience with basic obedience. Walking my two together, I often put them in sit- or down-stays when we meet other dogs and watch the amazed looks of the other dog owners as my dogs just calmly sit or lie there while the other dogs are yapping, growling, lunging, pulling, etc. It took a while (and a prong collar from this site <img src="/ubbthreads/images/graemlins/cool.gif" alt="" /> ) to get to that, but it seems to be working for now.
Enough blathering, though. I really enjoy my TM's. If you work with a good breeder to get the right puppy, are willing to take on the hassle of the amazingly destructive puppy and juvenile stages, work the heck out of them in obedience, and socialize with dogs and people like crazy (I guess that really applies to most any dog), you may enjoy a TM, too.
They are also great with kids -- we sit outside our local coffeeshop with the dogs, and every kid that comes along wants to pet them, and the dogs tolerate it very well.
Most breeders produce one or only a few litters a year, and I expect that most puppies are spoken for by now as the TM is in season only once a year in late fall or early winter.
Here is a link to some pictures of
Elvis (StormnAngels Canadian Platinum), taken at about 8 months. My girl, Misty (Himalaya's Valkyrie Mist) is a black and tan, simlar in coloration to
Shyama.
Hope this helps.