"Tetra" is a numerical prefix that means "four." The name "Tetrada" isn't any more a trademark infringement than "tetrahedron" would be, unless you really think '-da' is closer to '-is' than '-hedron'.
Game rules are not copyrightable, but the presentation of the game is. It doesn't take a lot of work to avoid copying a presentation, since all you have to do is develop your own artwork and game layout. I don't have a WP7 and I haven't looked at screenshots, but I'm (perhaps naively) trusting that the developer did this.
The only other copyright the Tetris company could hope to leverage would be on the code itself, which is incredibly unlikely because the formula for Tetris is simple and fairly transparent (compared to, say, the AI decisions in Warcraft).
The Tetris Company really doesn't have any recourse, and I'm enough of a bastard that I would simply reply to the takedown letter telling them to pound sand.