Yea they fit, and always have. One of my dealers tried telling me the thing about the attachments not fitting because the shafts are different sizes. Maybe that's something Stihl tells them, or maybe they just figure that since Stihl doesn't recommend it, and the 250 has more power than all the other trimmers. I dunno how that theory got its start.
I do know that they recommend the tiller attachment for the 250 and it says so in their catalog and on-line, so I e-mailed Stihl and asked why the others weren't, and if it was a power related issue. The reply I got said, that I was correct, it is just a power thing. The 250 has enough to destroy the attachments. I doubt under normal use or care it would, but if a guy lets employees run the tools, well, you know how that can go.