Just curious, what is the mashup you're working on? When researching historical places, I've often wished that Google Maps natively supported a 4th dimension so that changes over time could be recorded. Everything from "I want to see the businesses on this street as of 2001" to "Zoom out and see European boundaries as of 1100".
> 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 :)