Hello! I posted the following about my son's in-home daycare provider. She is not in S. Arlington, though; she's in Alexandria. We live in S. Arlington, and it takes us just 5-8 minutes to get to her house.
http://www.dcurbanmom.com/jforum/posts/list/462203.page
We absolutely love Monwara. She usually cares for fewer than the 5 children she is licensed for (by choice), so there is usually an opening. Right now she is caring for my son and another toddler, and if I had to guess, the 2 children will likely be moving on to daycare/preschool in a year, so that is another reason why she may have an opening when you are ready for childcare. In terms of costs, the cost varies depending on how many days/hours we send our son, as Monwara allows us to be flexible with scheduling. Currently he is in her care for 38 hours per week, and we pay $300.
I've also heard great things about Ruth Lopez, who is near Fairlington. I did not meet with her in person, as she did not have any openings when I was looking for childcare.
In-home daycare providers don't usually know whether they will have a spot available until a few weeks prior. I think Ruth Lopez may have a waitlist, though, so it may make sense to call her ahead of time. And I know Monwara has held spots for children in the past with a deposit.
Good luck!