I'm interested in possibly writing an aggregation application for subject/tags interested in.
Will Stack Overload be providing some sort of API for the public?
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You can download the Stack Overflow Data Dump under a Creative Commons license, and query the entire thing yourself.
As mentioned in the article, it's anonymised, and by its nature it won't be up to date, but it'll certainly give you the ability to run your own stats :)
Specifically for tags: You can subscribe to the RSS feed for particular tags. Here's the one for the "python" tag, for example:
Just click on the tag you're interested in, and then on the RSS symbol alt text http://sstatic.net/so/Img/feed-icon-small.png.