Re-read your posts of the past few days, in this thread and the "AREF bypass capacitance" thread. Fair enough, you haven't explicitly claimed that FPGA's are the best solution in all ways for all problems - I exaggerated. But you have made still a range of absurd claims about how good they are in many circumstances, despite evidence to the contrary.
FPGAs have their uses - there are things you can do with them that would be near-impossible, and very expensive, to do in any other way. And there is an overlap of problems that can be solved by either processors/microcontrollers or FPGAs. But face facts - there are lots of problems that are more efficiently done in software, and that means a microcontroller or processor (or soft processor /if/ you already need an FPGA for other things).
You will be a better advocate of FPGAs if you are realistic about them - at the moment, you are scaring off the fence-sitters.
I haven't implemented a Kalman filter myself, though I trust Tim's judgement here. But you don't need any experience to look at the Wikipedia page (or any other webpage or book) and see that this is a complex algorithm, and is best done step by step.
What you seem to be missing entirely here is that no one is saying Kalman filters cannot be implemented on FPGAs - we are saying it is vastly more difficult to do so. It takes a lot of work to learn to understand these things, and to test and debug the code step by step. It is orders of magnitude easier with software that is easy to start and stop, debug, view data, print out logs of data, feed with test data, run on a PC rather than the target, etc. (And if you think to mention FPGA "simulation" - or even "co-simulation" - don't bother, for reasons that are obvious to everyone else. If you want to talk about MyHDL or Lava, I'll be happy to hear your new ideas.)
When you have a good, working Kalman implementation in software, and you need to run it 100 times faster with little regard for hardware costs - /then/ it is time to break out the FPGA and transfer it over.
Yes, people implemented Kalman filters in FPGAs before there were hard ARM cores - there were other hard cpu cores before that (PowerPC, and older weaker ARMs) as well as a multitude of soft cpu cores. And as I say, it /is/ possible to implement Kalman in "pure" fpga. People implement these things for a doctoral thesis - while in the pure software world, people knock them up in a couple of days using software downloaded from the net. /That/ is the difference.