You will need a smaller mountain bike than road bike as you generally want a bit more standover height and it's really hard to get the front wheel off the ground if the top-tube is too long. Most companies go down to about a 15 inch frame: if I remember correctly, you're not that tall, but if you're a real shortie you might want to look at Gary Fisher, Giant, Kona and Marin as they are all decent enough brands that make extra small frames. You're shopping at a good time of year - the sales should be coming up!
top tube lenght is critical, she is right about that. You do not want to feel like you are in a road position in terms of being laid out. There are 13" mtb's out there too.