nickb.
My wife and I will be in Arillas the first week in September 2007, we also wish to get to San Stefanos.
The distance from Arillas to San Stefanos is a walkable distance by the main road, or eroding coastal path.
But! My wife and I may not be mobile enough to do that, the Taxi fare looks very costly.
An alternative, is the Water Taxi. I'm still trying to find more info about it, in terms of schedule and prices.
Hope this helps?