A low-lying placenta is called Placenta Previa. In complete placenta previa the placenta covers the cervix, meaning it would have to deliver before the baby, which obviously causes lots of problems. Placenta previa comes in complete (requiring a C-section), partial (which only sometimes requires a c-section), marginal (which almost never requires a C-section) and just a low-lying placenta (which is never an indicator for C-section alone).
I don't know the recurrent rate of it, but it is a fairly uncommon condition, and the placenta almost always is marginal or just low-lying at the time of delivery. Complete and partial placenta previa are very rare. If it worries you, you have a couple of options. You can try to hear where the placenta is with a doppler or a fetoscope, and if it is high, no problems. If it is low, or cannot be heard, you can have an ultrasound to confirm location. I don't think you have to worry :)
at 12:36 AM on Feb. 25, 2010