Intellectual property is a tough area in software and game development--often the lines are blurred and depending on how fierce the IP owner is, enforcement has a huge ranged from nothing too stringent.
Windows Phone developer Marios Karagiannis who has published the popular 'Monster Up' game has another in the Marketplace called 'Tetrada' which yes, is very much a Tetris clone. He just received a C&D letter from "Norris McLaughlin & Marcus, P.A. Attorneys at Law, on behalf of the Tetris Company asking us to remove Tetrada from the Marketplace because they say it looks too similar to their game and violates the copyright of Tetris (TM)."
Marios, being an indie, student developer is of course complying as he has no resources to take on such a company--who has been vehement about enforcing their IP of the Tetris brand on other platforms. The question is, of course, what does the Tetris company have a claim on? The name? The logo or the actual game play? We're not sure, but Marios is under the impression it is just the name/logo, not the game play itself, in which case his game does not violate their copyright.
The game is being pulled as we speak from the Marketplace and had fetched for $0.99. We bought it anyways, because hey, these things are now like collector items. Having said that, it's actually a darn nice game--outstanding graphics, good UI and a pleasure to play. And that's probably the rub here: this game was better than the Tetris brand on Windows Phone (see review).
Either way, consider this fair warning for other Tetris clones in the Marketplace, of which there seem to be quite a few.
While we won't take a position on the merits of the IP dispute here (you can do that in comments if you wish) you can read the developer's thoughts here on his blog. (And if you want to try and snag Tetrada while you can, it's here).