No, there is not a lake on Mars
While I was traveling around over the past couple weeks, I heard some snippets about a lake being found on Mars. I only just now have had the chance to track down the story [example], which perhaps is now old news. Here's the scoop: a team at University of Colorado, Boulder published a study with some details of a large lake on Mars that dried up about 300 million years ago. The surprising part is that this is much later than lakes are expected to occur on Mars. It's a terrific find, and appears to be a good target for looking for fossilized life.
The news reports appear to have been very, very skewed and misleading. Including the picture that I'll try to put here (might die if the link goes bad). It's of a partly filled lake in a Martian-like setting. Even on Discovery.com this was presented without explanation (unless you actually click on the image and read the caption). It's not cool.