UberMedia, the company that has been buying up Twitter apps including UberTwitter and EchoFon, is making its biggest catch yet — TweetDeck.

The acquisition was first reported in TechCrunch, and was later confirmed by All Things Digital’s Peter Kafka, who said that UberMedia will pay $30 million in cash and stock. He said the deal isn’t complete (which is presumably why neither company has announced it), but “it’s pretty far along, with signed term sheets, etc.”

This seems to solidify UberMedia’s plan to become the biggest player in the Twitter ecosystem — or as chief executive Bill Gross described his goal last month, “the best partner to Twitter in enhancing the Twitter ecosystem.”

It’s interesting to look back at a list of apps that Twitter published last September. Among the apps not developed by Twitter itself, TweetDeck was the most popular, except for Twitpic (which is a photo-sharing service, not an app for accessing Twitter). And of the top five third-party apps on that list, UberMedia now owns three.

At the time, VentureBeat’s Owen Thomas used the list to argue that external third-party developers aren’t particular important to Twitter anymore. However, if a single company controls most of the big apps (Loic Le Meur of competitor Seesmic says UberMedia’s apps are now responsible for 20 percent of all daily tweets), that equation starts to change.

(Update: Owen notes in the comments that I’m conflating two different metrics here. The Twitter blog post referred to unique users, while Le Meur is talking about tweet volume. An application could have be responsible for a large amount of tweets relative to its user numbers if they’re all “power users”.)

London-based TweetDeck raised less than $5 million in funding from Betaworks (also an UberMedia investor) and others.