Google has announced in a post on its Lat Long mapping blog that it's integrating Picasa Web Albums into Street View. The move means that photos taken by "millions of people" will be available to people exploring landmarks on Google Maps.
For pictures to be eligible, they'll need to be geo-tagged and available for public view. The catalogue of photos that pass through that filter will be joining that of Panoramio, which was added to the Street View service earlier in the year. Face detection will be applied to remove identifable individuals.
The features is live now, and can be seen at a number of landmarks across the world. Google suggests you check it out in Paris, Tokyo and New Zealand. Try Big Ben as well, with the recently added photozooming feature.