deCarta, formerly Telcontar, is talking about mapping mashups. Check out their developer zone.
Client side mashups as the “main approach”, with pushpins on locations in your data set.
They are experimenting with many advanced applications and these complex data mashups tend to be better run as server-side.
Map as picture vs map as … data. Customizing how the map is drawn is a new developer tool and a valuable one. Map for a game would have different look than the same map for a driving tour.
Given the very high quality of Google, MSN, Yahoo maps I’m not clear if they won’t become yet another victim of the giant .coms sweeping away early niche adopters, but hopefully they can remain competitive with free offerings to developers as here.
Joe Duck 
Joe,
I just want to point out that we (deCarta) are providing software to Google, Yahoo, Ask and a number of web (and mobile) mapping providers. As long as Google & al like our software and keep using it for their mapping solutions, we should be in good shape. Of course, as a software provider, we’re also interested in selling it to anyone and everyones who sees values in it. Our online web service is just another way of delivering it.
Thanks Louis – glad to hear it and good luck with your excellent work.