> Just curious, what is the mashup you're working on?
Putting on the map the villages of Wallachia (http://en.wikipedia.org/wiki/Wallachia), a historic Romanian province, by order of when they were first documented in the official archives. The project sits here: http://sate.maglina.ro/ , it's based on Django+Solr (there's a "Filter" menu at the top right), it's in Romanian, of course, but it's more or less self-explanatory.
I also created a couple of (I'd say cool :) ) heatmaps based on the data, which you can see here: http://sate.maglina.ro/heatmaps/ , giving a general idea where the early medieval settlements had a greater density. (for the heatmaps I used gheat, of course, http://code.google.com/p/gheat/ , which is a very, very cool project).
> I've often wished that Google Maps natively supported a 4th dimension so that changes over time could be recorded.
I feel you on that :) But, I'll have to admit, it could turn into a very controversial thing. For example, one of the reasons for which I started this small project of mine is that lots of historians (Hungarians and Romanians, mostly) have debated whether present-day Wallachia was inhabited by Romanians continuously (as the Romanian historians insist that it happened) or if said Romanians emigrated to Wallachia from South of the Danube somewhere around 1100-1200 or later (the Hungarians' view on the whole story). I was hoping that by putting on a map the Wallachian villages by order of them being officially mentioned it could clear up some things, at least for me :)