To add to what Jane said; in Arlington you have, by default, one of the key variables in real estate value which is scarcity. Such a small portion of land nestled right up against DC is always going to be in demand. As far as Boomers moving South, I agree with Jane that more people than many folks realize are actually heading toward Arlington for better convenience to all the area has to offer. Also, don't under estimate who is buying the homes and in what tier. In many cases a 600k to 700k purchase is a starter home in Arlington these days. The 1m plus homes are being picked up by folks in their mid to late 30's and into 40's who are looking for more space for their families. There is plenty of demand to scoop up what boomers might leave behind.
Even right now, more inventory would be moving if the loan standards were not so high for Jumbo mortgages. What I did not mention above, is that today's buyers have much higher expectations for what a home offers, and many neighborhoods in Arlington present an unique opportunity to level the current home (a minimal portion of the total value) and build to suit on the site. I love the look of these older, more mature neighborhoods, but the reality is they homes are small, with very small bedrooms, and often only one bathroom. The next generation, heck the current generation, of homeowner is not going to be satisfied with that product.