I produced a quick colour rough with my faithful wacom in Photoshop, DWM approved, then I did some finer, "proper" figure drawing type stuff (also in Photoshop) before exporting it over to Illustrator. I should point out that the folks at DWM, aware of my various styles, asked for something not too cartoony.
In Illustrator I just draw over the top of the image freehand using the pencil tool with the stroke set to zero. Most illustrators use the vector tools to make perfect shapes. The truth is I didn't understand how to use illustrator when I started (still don't!) and so picked up the pencil tool and started drawing shapes of colour and sticking them together. The Monk and Cat image from the Guardian that sits in the top right corner of this blog page is an early example of my Adobe Illustrator stuff.
I'll be starting on my next spot illo for DWM soon (a Colin Baker one! Eek!), I've had the brief for a couple of weeks, but I haven't got a clue what I'm going to do with it yet. It'll certainly be done in Illustrator though, I love the way it leaves drawing looking so immediate and fresh. Perfect for spot art that should have impact without becoming dominant and distracting. Basically Adrian Salmon's Time Team illustrations are perfect for the magazine in that respect, mine are attempting to do a similar job.