According to All About Phones, a Dutch website, Google Maps Navigation will soon have a full-blown offline mode. Late last year, with the introduction of Google Maps 5.0 for Android, GMN moved to vector graphics and improved route caching. But this new capability would allow you to plug in a destination while outside of cell range, which means that some maps and associated data would always be cached on your device.
Citing an unnamed Android phone manufacturer, All About Phones says you can expect this new feature sometime this summer.
Via Android and Me