Once you get beyond the most basic map mashups, you'll run into a problem with data storage and access, because you can't hard-code hundreds of markers. It's a problem that SimpleGeo wants to fix. And from the looks of their new platform, they've gone beyond expectations.
Have you noticed the many tools to share your location? There's FourSquare and Gowalla, both about a year old. Then there are old-timers like Brightkite. And now, there's one that works with all three (and a couple more).