convert taking a grayscale image and turning it into RGB

I just ran into this problem and can't seem to convince myself that it is
not a bug.

Here's what's happening:

I have a .png image that is grayscale:
$ file mottle.png
mottle.png: PNG image data, 200 x 200, 8-bit grayscale, non-interlaced
$ identify mottle.png
mottle.png PNG 200x200 200x200+0+0 PseudoClass 256c 8-bit 6.18359kb

Now I want to run it through convert and get a pnm:
$ convert mottle.png mottle.pnm
$ file mottle.pnm
mottle.pnm: Netpbm PPM "rawbits" image data
$ identify mottle.pnm
mottle.pnm PNM 200x200 200x200+0+0 DirectClass 8-bit 117.225kb

And by the way:
$ convert -version
Version: ImageMagick 6.4.1 05/02/08 Q16 http://www.imagemagick.org

The pnm is RGB! What happened here? Most interestingly an older version of
convert works fine:
$ convert -version
Version: ImageMagick 6.2.4 10/02/07 Q16 http://www.imagemagick.org

$ convert mottle.pnm mottle2.pnm
$ file mottle2.pnm
mottle2.pnm: Netpbm PGM "rawbits" image data
$ identify mottle2.pnm
mottle2.pnm PNM 200x200 PseudoClass 256c 39kb

Is this correct behavior for the new version or was a bug introduced
somewhere in the line of development?

-- 
Viktor R. Ivanov <[email protected]>