Isn't this exactly what Twitter said they DON'T want to see (replacement clients using the Twitter API). Is TweetDeck just "grandfathered in"?
Twitter: Want unlimited API calls?
TweetDeck: YES!
Twitter: Okay, thats $XX,XXX a year...http://socialmediatoday.com/imckeevocaniccom/282916/how-many...
http://groups.google.com/group/twitter-development-talk/brow...
It seems stronger than merely a "recommendation" to me. They flatly state that new apps may not replicate the mainstream Twitter experience. Existing developers are allowed to continue to "serve their customers" - but I think developing new product lines as TweetDeck is doing, is going to run afoul of the new API terms of service.
Unless they have a side agreement, I expect some sort of battle to arise from this.
Though there is always the chance that twitter tries to buy tweetdeck.