Superfeedr, the real-time feed transformation service funded by Betaworks and Mark Cuban, announced today that it now offers real-time keyword tracking across all 2.1 million feeds that are run through its hub.
Any time your keywords of interest appear across Tumblr, Posterous, Typepad, Gowalla, Gawker, HuffingtonPost or a number of other sources subscribed to Superfeedr (including ReadWriteWeb!) you can receive real-time notification pushed to you within seconds.
That's the theory anyway. In reality, Superfeedr will push items to you as fast as it gets them - but any number of things can delay the publication flow along the way.
Superfeedr also chose today to announce that it will begin charging developers for notifications after their apps receive over 25,000 pushed to them. You can see sample apps on a webpage (RSSFall, from the guy who made Twitterfall), via IM (at Supertrackr) or on FriendFeed.
Real-time keyword tracking as a service could prove valuable to all kinds of developers. Is a 2.1 million pool of source feeds big or small? That's hard to say. Depending on your keyword, it can deliver a lot of data - but it's such a sliver of the social web that it's hard to know where to place this in the bigger picture.
This is probably most useful as a developers' proof of concept, but give it a try - end users may be able to get some value out of it as well.
See also: the Collecta API. Collecta does something very similar and has relationships with some key publishers, like WordPress, that Superfeedr does not. Collecta also includes Twitter messages.
Real-time pushed keyword search from the web is definitely a viable asset to leverage these days.