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Google Maps Guide Tom replied to a Google Groups thread requesting Google to bridge the data between My Maps and Google Mobile Maps.
Typically, a Google representative would say, thank you for your feedback and leave it at that. But Tom said, "keep checking back for updates!" That implies to me that this feature might be coming in the next release.
So soon, if you save maps on My Maps, then you might be able to take them with you on your Mobile device.
As an FYI, the new iPhone 2.2 update has new map features such as street views, new pin placements, walking and transit directions and more.
Forum discussion at Google Groups.
