Real estate website Idealista seems to be the first Spanish website to add Street View to their maps.
Real estate website fotocasa have also added the Street View layer to their Google Maps but don't seem to have added the peg man icon. Therefore it isn't yet possible to check the properties on their site using Google's panoramic images.
Idealistsa, however, have got a working demo together in their 'labs' section. Street View works very well in real estate sites, as it gives customers the opportunity to view the house and surrounding area virtually before viewing for real. As Idealista say you can use Street View to "move around (and) learn more about the area."
I'm sure more Spanish websites will soon follow Idealista's lead and add Street View to their Google Maps mash-ups.