It is expected that just in a few more years, most cameras may come with GPS and all photos will have the location, latitude and logitude, in their metadata such as EXIF. This will help in organizing photos. This may also help a lot more in analyzing photos and organizing them meaningfully for retrieval. It is clear that time and location are two major factors that will help in retrieving personal photos. For retrieving other people’s [photos or collections, one may need to use other factors.
Though the location is not yet available in most cameras, the usefulness of location has been well recognized. Many photo sites, including Flickr and Picasa, provide facilities to do geotagging. Using this facility, you can place your photos on a specific location on a map and then browse/retrieve photos based on their location.
A good review of this feature, commonly called geotagging, is given here. Not surprisingly, Flickr does the best — they have been doing this for the longest time.