In fact, when I first started this project, I had called it "Tetr61s". This was because the game was at first supposed to be quite close to Tetris(c) and there's a letter in Russian which is pronounced "i" (like in "tetris") and when you write it down it looks like you have written 61.
Since the Tetris Company (http://www.tetris.com) claims to have a look-and-feel copyright on Tetris(c), and sues independant game programmers who program Tetris(c) like games, I could not call the game Tetr61s, nor make a plain Tetris(c) clone. So I decided to start a new project, more general, and change the name. So "U" is because my nickname is "U-Foot" and "61" is because it is the successor of "Tetr61s".
Yes, since the game is Free Software (free as in speech), protected by the GPL (GNU General Public License). You should have received a copy of the GPL with the game. For more informations on the GPL, check "http://www.gnu.org".