These guys at Custom-Tradingcards.com seem set up to do fairly inexpensive printing of cards, although their focus is definitely on trading cards rather than playing cards. Their prices get much better with bigger print runs, as most of these places do. The default pricing quote gizmo on their website gives prices not too far off what I'm getting from printers for my card game for big print runs, and they seem to have customizable templates and are prepped for user-submitted art.
Unlike the other places I've linked to here, they seem to be able to do randomized collating and small-pack packaging, too, which would be a big deal for those developing collectible card games like Magic: The Gathering or Pokemon. I don't have a good idea of what their product is like, and I haven't used them, but I thought I'd add the link in case any of my mighty and burgeoning readership is looking for this kind of service.