In the new version of Google Maps (5.7) for Android you can download maps to your phone.
Tap menu > More > Labs. Select Download map area. Once you’ve done that, just follow the directions: “To download an area of the map, long press on the center of the area you’d like to save, then tap on the bubble. Or go to ‘More options’ on a search result details page. Then select ‘Download map area’.