Agree with Rosetreader. Normal prognosis? Depends who you ask. It is *traditionally* looked upon as an inexplainable & incurable condition that can only be managed palliatively until it progresses to such a state that the horse cannot be made comfortable, or the hooves completely break down.
I also agree that it is *likely* not genetic, except in that some hoof conformation types, such as typical QH or arab hooves are more likely to develop these probs *when not managed well*. I think of no. 1 importance is management & ensuring horses get a LOT(compared to normal) of exercise on firm ground from a very young age. Trimming is also very important, and farriers also often tend to trim certain breeds or types in a certain manner - eg. QH's often left with high heels, which makes them more prone to 'navicular' issues.
Basing my attitude on recent research, and experience in successfully rehabilitating some of these horses(along with other 'incurable' conditions such as founder with sole penetration), it seems that the prob is, most domestic horses have underdeveloped digital cushions & lateral cartilages. They don't get enough exercise & good hoofcare from birth, so the soft, fatty digital cushion that is quite adequate to support the weight of a foal, never develops into something that can support a grown horse. This causes them not to be strong enough for heel first impact. Bad trimming, diet & thrush can exacerbate the issue, until the horse is very sensitive in his (underused) heels. I think 'navicular syndrome'(ie unexplained heel pain) is much more common than people suspect, and is probably the more usual cause of horses being 'ouchy' on gravel & rocks, rather than thin or weak soles, which are what is usually blamed.
Unfortunately the traditional palliative measures of bar shoes, pads, wedges etc. further remove the heels from use & therefore cause them to become weaker, along with forcing the horse more on his toes, which create other probs including damage to the ddft & bones of the hoof & leg(not just nav bone). It seems to be the regular state of affairs that horses managed palliatively in this manner can generally be kept relatively comfortable(but generally unable to 'work') even when quite advanced, until due to the continued toe first impacts, the horse founders badly(I've seen horses with *convex* soles!), and that is generally the end of the road.
As you may be coming to see, the horse desperately needs to use his heels to be/become sound & develop strong feet, and the above does virtually the exact opposite. But just shearing high heels down to 'correct' parametres isn't likely to be appropriate treatment, especially in the beginning, because it will just make the horse more uncomfortable, less likely to get the much needed exercise, and he will still likely 'tippy toe' due to pain regardless how his feet are trimmed. So my approach is *generally* to lower heels only gradually, and protect sensitive heels with boots &/or pads, but ensure his heels aren't too out of the picture to get stimulation from ground pressure. Eg. If they're still high, use frog support pads for extra pressure.
In addition to Rose's links, hoofrehab.com has some great info & I highly recommend Pete's book(altho he wrote it some time ago & has better, more precise info on some things now), and if you have the money, his DVD set. You will find plenty of other links on his site and plenty of other sources of good info online. There're also the studies done by Dr Chris Pollitt(sp?) of the University of Queensland, but I can't find the articles or my links to studies since my computer crashed a couple of months back.