So now burger joints Carl's Jr. and Hardee's, both units of CKE Restaurants (NYSE:CKR), are offering bunless, lettuce-wrapped burgers? That's certainly news to those on the low-carbohydrate Atkins and South Beach diets, or those wanting to shed 40 pounds. Jay Leno and David Letterman must be licking their chops.

Don't laugh. Carl's Jr.'s bunless burger is a 1/2-pound seasoned and charbroiled hamburger patty, wrapped in iceberg lettuce leaves, with mustard, mayo, cheese, tomato, red onion, a reduced amount of ketchup, and dill pickles.

It -- and I don't use the word "it" lightly -- is half-wrapped in paper and served in a cardboard carton. The result is 55 fewer grams of carbohydrates (not counting the cardboard) at $3.95 a pop. The Hardee's version, a 1/3-pound Angus beef burger patty, goes for $2.89.

CKE has been steaking out the high-end hamburger market for years. Hardee's Thickburger and Carl Jr.'s Six Dollar Burger (appropriately priced at $3.95) are designed to "deliver on quality and taste." Too bad Yum! Brands' (NYSE:YUM) Taco Bell already scarfed up the slogan "Think outside the bun."

Too bad, too, that competitors McDonald's (NYSE:MCD) and Wendy's (NYSE:WEN) are enjoying double-digit sales growth. CKE, struggling to catch the wave, recently announced sales for Carl's Jr. up a meager 3.1% and Hardees, up 6.4%.

Perhaps more pressing, profitability is downright anorexic at CKE. Operating margins of 2.2% are dwarfed by the 12.3% margins enjoyed by leading restaurant owner Yum! and the 13%-plus margins turned in by McDoanld's and Wendy's.

And yet, for whatever reason, investors are biting. CKE, a $40 stock in 1998, now trades at $5.90, three times its lows of $1.94 back in 2000. Someone's clearly counting on the company converting higher prices and product innovations into free cash flow. Thus far, management has proven it can do neither -- high or low carb.

If you ask us, "thinking outside the bun" might mean dumping the paper and lettuce wraps in favor of a knife and fork. Dieters want good food -- and a lettuce wrap is something you'd find at a spa. If McDonald's can serve up hotcakes on a plate -- not to mention double-digit margins -- why not serve a bunless burger on one, too?

W.D. Crotty owns stock in McDonald's and Yum! Brands. Join W.D. on the Motley Fool's CKE discussion board to evaluate CKE and the bunless burger. For a 30-day free trial to the discussion boards, click here.