Stuart, you’re a god. I want to create JPGs for large hi-def printing, (over 10k pixels square), and I’ve been plagued by the limitations of Fireworks and Illustrator. Here’s what I found:
Fireworks CS5 (yes I know it’s a web app, but you can really push it, and it’s fast to work in, with great filters) – I used 18 megapixels photos and worked at eighth scale – 1250 pixels square, then when finished, blew up to 10k x 10k using Image Size (it doesn’t like scaling symbols though)
Illustrator CS5 (just learning it now) – I unchecked the PDF-ready boxes and used just link files. Worked a treat – produced works 60″ by 40″ at 300dpi – that’s 18k x 9k pixels – using your mulitple-techniques. Superb! Really high def printed artwork for very close inspection by visitors.