I think they used to do this - I could swear that I used to have a third-party plugin that I "installed" on my Google account that would show me JavaDocs whenever I Googled a Java class name. It hasn't shown up in about 3 years, though, and I haven't found any trace of it in the code I work with (which should be right around that area of the page...), so maybe it was discontinued.
At the bottom is 'Subscribed Links'. I recall having the same java search thing enabled, but it looks like there's only a lame javacio.us thing now that doesn't work the same way. They still have a manpages search option which is nifty, though it doesn't show up at the top anymore, unfortunately.
It should not require you to subscribe. User feedback (possibly implicit) should be used to determine what third party code is used for which queries/contexts by default.
Such an API would allow third party developers to add Wolfram Alpha like features among other things right into the core Google search.