I've seen this question posted before and have wondered this myself as to whether or not lesions disappear. I thought this would be a good time to share my experience.
I just got my MRI results back, and most of the lesions have disappeared. I was diagnosed February 2008. I had lesions around the corpus callosum, cerebellum, frontal lobe, and smaller ones scattered in the other parts of my brain. The bigger one in the frontal lobe is the only one that is left. The others have gone bye-bye (at least they aren't seen). This would account for the improvement in my symptoms since starting Copaxone in March 2008. So, from first-hand experience, I can tell you that it does look like lesions can diseappear or get so small as to not be seen on the MRI.
I do want to add the symptoms haven't gone bye-bye (I wish they would), but I have improved significantly. I can walk without a lot of difficulty, which is a far cry from where I was over a year ago! Most symptoms come back when I'm really tired or hot. I can barely see when I'm too warm, and will trip and fall over my right foot. I only get tremors when I'm stressed out or tired, and I drop things from time to time. I guess the worst thing I've had is the stabbing head pain, but I haven't had problems with that in a week.
Hopefully, this information may be useful for someone. MS is pretty unpredictable. If you were to ask me if I was going to be in a wheelchair before treatment, I'd tell you I would. I feel very fortunate that I've gotten the right treatment.
Best wishes for others suffering from the disease,
Deb