An article I came across that addressed this issue (along with other interesting info) is at:http://www.lupinegames.com/articles/path_to_dev.html
The relevant section is:
Something I need to mention is that when you make your Tetris game, you can't call it "Tetris". Tetris is a trademark of the Tetris Company who is owned by Alexey Pajitnov, the creator of Tetris. It is his exclusive right to use the name Tetris, and I believe they may have won a lawsuit saying that you cannot make a falling blocks game with the syllable "tris", as it is obviously playing off the popularity of the of the name Tetris.
However, this means nothing to you if you call your game "The Sky is Falling", or anything without a "tris" in it, as they do NOT own the gameplay, interface, or idea of falling blocks. If you hear anything differently from anyone, tell them you can't own ideas, and if you require further proof you can look up information on this subject at the USPTO (http://www.uspto.gov/
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I read elsewhere about Hasbro very aggressively defending the copyrights on a bunch of "vintage" games it bought up the rights to, so you probably don't want to mess with them...