coffeeking, the Whyte fork did indeed have a J type axle path (of sorts), but this was designed into it. Seeing as the fork ran on linkages not dissimilar to the way in which a modern VPP frame works, you could design pretty much any axle path into it you desired, if starting with a clean sheet of paper that is.
Regarding the success and disappearance of Whyte's full sus bikes, I think (though can't say for sure) that the Whyte designs became a bit of a victim of their own success. Marins sold LOADS as soon as they'd taken Jon Whyte on to design a frame in around 96/97, and have continued to do so over the years. The problem to an extent is one of snobbery, and Marin's being fairly ubiquitous has meant that those seeking something of a higher price tag and more unique don't want to buy off the person who designs full sus bikes that sell for under Â£1k, if you see where I'm coming from.
There's also the total integration factor to take into account. People like to change/tinker/upgrade their equipment to suit. With the Whyte PRST design, that's it, you're stuck with the frame/fork combo. If it suits, great, if it doesn't you can't change it!
Another thing to consider is that from what I know, Jon Whyte was pro long travel suspension on XC bikes, so as soon as the ability to use a long travel yet lightweight fork became available (the Maverick DUC32), he designed a bike for XC use that would use it (The Whyte 46). This pretty much made the PRST obsolete, as it was as light, if not lighter, had adustable travel, was more conventional (better?) looking.
Then there was of course the image. This one I can definitely say is justified, if a little harsh, as every Whyte PRST I have EVER seen has been piloted by a man of at least 50 years of age. Sorry, but they were seen as an old man's bike, and that's just not cool!
The linkage fork thing is one of those things that has never been properly perfected really. In theory, there are many advantages, but the only people to have persevered with the design for more than a couple of years are BMW on their motorbikes. Others have tried, but none as successfully as BMW. And BMW motorbikes are VERY expensive. Conventional forks are much cheaper to make/design/install, and work perfectly well enough for 99.9999999% of the population! Put it this way, have you ever seen a linkage fork on a MotoGP bike, or a Motocross bike, or even any DH Mountain bike? All of which the suspension is much more critical on than an XC Mountain bike.
Regarding coffeking's comment about the rate of improvements happening, he's right to an extent, though things have happened a bit quicker on MTB's. From the mid 90's when the Proflex design with its elastomer springs was hi tech, to by Y2K where full sus bikes with 6 inches of usable travel or more both ends were available. Since then it's been more of a trickle effect, but just as when V brakes appeared, and then discs did, the instant improvement was great. Since then better brakes are on the market, but they're not significantly better compared to the distance between canti's/V brakes/Discs.
Trustyrusty. If you're looking for an old full sus frame to Singlespeed, forget the Klein Mantra (they were crap!) and look for an old Orange X1, or better (1 1/8 headset, not 1", and ally frame not cromo) an X2. Or possibly one of the Trek Y bikes.