I've been in charge of IT at my employer for the last eleven years, and that means I've been in charge of moving the infrastructure of HQ four times. (Cambridge office space is a vicious market.) In that time we lost no employees because of the moves.
I attribute this to a few simple things.
First, there is an understanding that the company's moves are dictated by the market. We seem to have a knack for picking spaces that will be much more valuable when our lease is up. In two cases, Microsoft has decided that the building we were in was perfect for them. If MS decides that they want to occupy a large chunk (or all) of a building, nobody smaller than Google can afford to counterbid.
Second, there is an understanding that the company will stay near the Red Line, somewhere north of South Station and possibly as far out as Alewife. In practice we've moved between Harvard, Central and Kendall. I think the best was at 1 Cambridge Center, which is built on top of the Kendall Square T station. MS has that now. They had a new exterior elevator built in an inconvenient location in order to have private elevator service...
Third, we haven't been victims of Whyte's Rule -- corporate headquarters always move to minimize the CEO's commute. Our president lives in Cambridge, and sometimes his commute gets a little longer and sometimes a little shorter.
To sum up, you can keep your employees happy even with frequent moves if you are consistent, open, and keep to the same general area.