You've probably discovered that your Dell 5100cn color laser doesn't play well with Snow Leopard. Before you list the 5100cn on Craigslist, try this:
Delete your old Dell printer in the Print & Fax System Preferences panel. Install a new printer, but select the Mac-installed Samsung CLP-770 Series PS, and presto! You are back in business, complete with duplex printing and toner supply levels. Sweet!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090829161726374