Another bleach-based idea that I found very effective. It is NOT Earth-friendly, however.
Wearing rubber gloves (oh boy, you can already tell this is going to be bad), dip a few paper towels in a mild bleach/water solution. Hang these against the wall where the gross grout is (for wall grout, obviously). Leave it. Come back in an hour or longer and the grout is gorgeous. All you have to do is spray off the disintergrated paper towel.
I like this better than using spray stuff which I feel like gets everywhere. This way I have much more control over the area.
Has anyone tried the vinegar/baking soda solution? If that works well, I'd probably prefer that. I'm trying to wean myself from bleach.