as far as I know, no plug-in has been written for GIMP to handle them. Am I wrong in this, or have I just not been able to find it?
I think that is correct.
A bit of a struggle finding anything for converting paint_dot_net files.
The usual xnview/irfan forums did not offer much hope. One telling line was ... "paint.net developed jointly with uni of 'something' and Micro$oft". Any file format + M$ = pita
Same thing with online convertors, I am sure you have already looked at these.
One small ray of hope, some command line converters from herehttp://www.comsquare.ch/index.php?option=com_content&task=view&id=36&Itemid=55
Thankfully these do not rely on the dot_net installations and even work in linux with WINE.
They are beta and there are caveats for pdn to bmp/png format, png works best.
I have tried the other way png -> pdn and trying out in a windows noteboot with PaintdotNet & AFAIK works ok. The snag with this is - no layers.
I've had a closer look at the read me files (never rtfm LOL) that come with the zips. It does look like you can save a multilayer pdn as separate images and conversely combine a set of separate images into a layered pdn. Worth giving it a try & if it works, perhaps setting up a batch file. There are gimp scripts that assist in saving layers as separate images. A good one is Saul Goodes script 'sg-save-all-layers.scm' - towards the end of this page.http://flashingtwelve.brickfilms.com/GIMP/Scripts/
best of luck