Google is releasing an application for the G1 that allows you to augment the Google Maps app with custom graphics and locations.

You can edit directly from an Android phone, and then sync with the online Google maps application, or vice versa. Since you can sync in both directions, you could potentially plan a trip, and then take pictures and upload them back into your Google maps, and then share that information.

Currently, it will only work with the T-Mobile G1, however that could change as other Android based phones are released.