I used ImageMagick to convert a PDF file only two days ago, with success. Wait, I'll try again...
Nope, no problem here. I can convert PDF to PNG, to JPG and to TIF, and the resizing works too.
It takes quite a while for long files though. Have you tried it with several different PDFs (perhaps generated by different programs, too)?
Edit: I'm on millhouse; don't know if that affects anything.