Epworth Preschool and Kindergarten is fantastic. It's part of a church, though. It's on the Gaithersburg/Rockville line.
http://www.epworthsteeple.org/ChristianEducationPreschl.html
Also love Christ Episcopal School in downtown Rockville. I have no idea if they have openings, though.
http://www.ces-rockville.org/
Thomas Jefferson Prep is a private pre-school - 8th grade, located in a church, but secular. It's Montessori inspired. It's in Gaithersburg, on Rt. 28/Darnestown Rd., very close to Rockville/North Potomac area. They are still enrolling.
http://www.tjprep.org/
I've heard excellent things about Franklin Montessori as well.
http://www.montessori-mmi.com/admission2.php
Our child is going to Seneca Academy this year, which I also love (of course I'm biased). It's further up in Darnestown. We live in Rockville and it's about a 15 minute commute. Not sure if they have openings, but it's worth looking into.
http://www.senecaacademy.org/