Google said that the Maps Data API is being deprecated and will no longer be available after Jan ’31, 2011.
“Maps Data API will continue to be accessible until Jan 31st and Maps created using Maps Data API will remain accessible in Google My Maps beyond this date. During the deprecation period we’re also providing a Maps Data API data liberation tool, that offers download of Maps from Maps Data API in KML format, or transfer to Fusion Tables, by the user that owns the data.
Both data transfer and download to KML will preserve all data for the vast majority of maps. There’re some rarely used features (e.g. certain custom properties) that’re not represented in the KML download,” explains Google.
See the FAQ in the tool for more details.