Pure brilliance. Google, you paying attention? This is awesome. Yahoo's just released what they're calling "MapMixer" - finally those campus/park/event specific maps locked away on websites can be overlaid on top of a bigger map to show context. I did my college campus. My second attempt, adding my former church in Pasadena, failed. Twice. But the potential of this is very cool. I suspect some Google and Microsoft programmers are gonna be extra busy the next couple of days playing catchup. Nice job, Yahoo! (I still hate your name and therefore refuse to use any of your offerings, but serious kudos on a most excellent leap forward.)
If I had one suggestion, it would be to allow you to pull from an existing URL, instead of having to download the image and then upload it again to your service.