Density option being ignored?

I'm using ImageMagick 6.4.4.6 on a Gentoo installation. One of my sites 
needs to take images uploaded by users in various formats and convert 
them to greyscale tiffs of a set size and resolution for use in print.

For some reason, no matter what I try I can't get the density option to 
work.

Is there any reason why the following shouldn't work?

convert origionalfile.jpg -colorspace Gray -thumbnail 181x181 -gravity 
center -background white -extent 181x181 -set density 200 outfile.tif

This gives me a greyscale image that is 181x181 pixels as desired but 
the resolution stays the same as origionalfile.jpg

this doesn't work either:

convert origionalfile.jpeg -colorspace Gray -thumbnail 181x181 -gravity 
center -background white -extent 181x181 outfile.tif
convert -density 200 outfile.tif outfile.tif

or this:

convert origionalfile.jpeg -colorspace Gray -thumbnail 181x181 -gravity 
center -background white -extent 181x181 outfile.tif
convert outfile.tif -set density 200 outfile.tif

I've tried all sorts of things but nothing seems to work.

-- 

Matt Fraser
IT Department
Allied Press Limited
+64-3-479-3505 ext. 8046
www.alliedpress.co.nz