I too haven’t heard of that but recently did a test on it and here is what I got:
I kept writing the letter Z until it reached 6000 characters and then posted it. After that I again tried it but this time reached 15000+ characters and still I could post my status. So I believe there is no limitation as of now but in near future, Facebook might follow Twitter and make a limitation of 140 characters for your status updates and wall postings.
This is just my personal thought, so no hard feelings. What do you think of this?