Here's a bit about the lettuce in question.
The NASA experiment, called Veg-01, will help scientists determine the feasibility of fresh-grown food for long-term space missions, like ones that would send astronauts to Mars. This isn't the first batch of lettuce grown. The first round of romaine was sent down to Earth and tested for safety. So, no, we shouldn't be worried that the space lettuce will do anything freaky to our dear astronauts.
Half the new crop will be served up with some oil and vinegar. Space food has improved a lot over the past few decades, but there's no doubt that some fresh, crisp lettuce will be a rare and exciting treat.
Some day soon, astronauts could count on a more ready supply of these space veggies.