Top out.... what a BI*C*........
Everyone will experience this sometime in their riding life...
I have had this problem with my Stratos MX 6's and my Rockshox Boxxer teams....

Some reasons for top out:

1. All the internals are not in the best condition
2. Your pistons in the fork are not the correct size, could be off by acouple of mm (May have shaved themself's like a mates RST's)
3. You are not running the correct amount or weight of oil in the side of the fork with the dampening rod
4. Internals have not been put together properly

These are just SOME of the reasons for top out...
Sometimes cheap forks do it and sometimes you get a dodgy set from a main brand eg. rockshox and marzochi.

If you are willing to rebuild your fork that is what I would do or take them down to your LBS (does LBS count as aol talk and not being alowed?) and get them to rebuild them.

Hope you find the problem and fix it. Keep us posted becuase everyone gets top out..