There was a thread not too long ago about Tiger trout. One of the reasons the state quit their Tiger trout program was the low hatch rate. So they are basically doomed from the start in the wild.
That's probably not entirely true. They are no more doomed then any other trout from the beginning once born. Keep in mind that any trout born in the wild has a nearly impossible shot of making it to the 7-10" range so naturally any wild tiger trout making it to that size is ultra rare. Just how nature works. That doesn't make them any less capable of survival as they are gifted with the same skills as their parents to survive. Of the 8 wild tigers I've caught they have all looked plump, happy, and healthy! I do believe that the fertilized egg only has something like a 5-10% survival rate for tigers unless heat shocked but I could be wrong with that, making it even more amazing to catch one.
You wont find much info on their rarity. At least not statistically speaking. You have to have a stream with both wild browns and native brooks but the ratio doesn't seem to matter (at least for the places I've caught them in). Two years back I caught a wild tiger from a stream that I have never seen nor heard of a native brook trout being in but I'll bet in the headwaters they are there in small numbers. In reality all it takes is for one female brown to drop eggs and have the brookie male accidently fertilize them with close redds. The odds of catching one are just rediculous.
I once lost one I nearly had in my hands. You'd have thought I had just lost a 30" bruiser. I wanted to fall to my knees and cry. To me it is the greatest trophy and accomplishment in all of trout fishing. It's awesome because you've just landed a fish that 99.999% of all other trout anglers will never get to catch. It's a special thing.