Is the filedata field in the thumb table blank? Trying to figure out if the error is in making the thumbs or just displaying them.

You can also try remarking out this line in functions.php:

unlink($thumbname);

Just put a // at the start of the line.

Then make a dumby post, with a BMP/JPG/GIF attachment. If ImageMagick part is working, it'll create a file:
/tmppath/th_[randomstuff].

/tmppath is most likely /tmp, but may vary.