I'm getting ready to publish my first collection of strips, too. Right now, I'm going with Lulu.com They specialize in stuff like books, images, and CDs and have a better pricing system than Cafepress. They also offer full color (Albeit at a higher price) instead of just black and white. Plan 9 does print a lot of comics I read, but in order to submit to them you have to have your work in print format anyway because they like it in hardcopy. You'd probably just be better off printing through Lulu first and sending a copy to Plan 9.

I've never used lulu.com but it looks really interesting. The pricing is decent especially compared with cafepress (except that lulu takes 20% of your profit in addition to any base price/cost), but the most interesting thing to me was it lets you simultaneously publish in print, on cd and via download, with no base price that I could see for the download version. So if you want to, you can publish the downloadable version for free, or an abridged downloadable version for free and then charge for it in print. If you've got a website with decent traffic, I'd expect you could sell a decent number of copies just pointing people to your lulu page(s) (not that I've done it before).