According to some it's the most likely option. Part of the problem for the Leave campaign is that it's really not clear what an "out" Britain will look like. I think leaving but then rejoining the EEA will simply reduce our say in important matters that affect us since we'll bound by a host of rules that we neither have direct democratic influence of nor are negotiated by our government.
The problem for the Remain campaign is that it's not clear what an in Britain will look like either. The words "ever closer union" are etched in the EU's DNA, it's clear that staying as we are today is not an option. If we stay we'll see more and more powers transferred to Brussels, more and more people from eastern Europe moving to the UK (Bosnia applied to join the EU yesterday).
There is no option on the referendum ballot paper for the status quo.