Well yes! Twice, once in 2007 and once in 2008 i went to Portugal for 6 months and 3 months so only purchased one way tickets as i was unsure of return dates.
EasyJet treat any return as two singles, each leg being priced separately. You don't get a discount on the whole trip by booking 'return'.It's always whatever two singles cost.Therefore they advertise the cheapest outward fare from the UK.
Ryanair do the same.