I'm amazed how well CUPS works these days. As soon as the packages were installed on my N900, I connected to wireless and found my server (CUPS 1.4.4, Debian squeeze) without even the slightest trouble. It just worked, without any configuration on my part outside of demolition's instructions!

lpr makes a great printer for text files and pdfs, which print exactly as I would have intended. Images are a little trickier though. I found that opening images (from either File Browser or Modest) and resizing them makes them print well enough on a single page. For US letter-size paper, setting a 2400x3000 to 40% of its original size (960x1200) will use most of the page capacity.

Ideally CUPS lpr would offer to scale images down to fit on a single page, but that is an upstream feature request. Many thanks to mase and everyone who cared enough to make N900 printing work!