When Glyph named TX-theory, he did not specify what the "TX" stood for, presumably because he did not feel he had the right to name a theory which he had not been able to fully describe. According to Glyph himself:
"TX" stands for "TwistedmatriX", "Transmit", or "Twisted multipleXed, according to taste.
However, as presented in the upcoming docudrama by producer Chris "radix" Armstrong and director Duncan "oubiwann" McGreggor, "TX" stands for "Twisted Extensions," though this is also contended, sometimes by Chris himself who has already pitched a counter-docudrama (to an un-named Hollywood backer) focusing entirely on the non-extensionness of TX-theory.
Cynics have noted that the "TX" could be rot-13 of Glyph Lefkowitz, in an alternate English alphabet with an additional letter inserted between those of "T" and "X." Even more insidious are the rumors that "TX" stands for "Prophecy Blade Epic Destiny Quest Adventure."
From the definitive history (as yet unwritten, but leaked by the time-traveling nanobots whom we all will one day serve) on the matter:
Apologies to the author(s) of the M-Threory article on Wikipedia. For a slightly more real take, read the Labs' announcement.
The name TX-theory is slightly ambiguous. It can be used to refer to both the particular seventeen-dimensional Twisted construct that Glyph originally proposed, or it can be used to refer to a kind of theory which looks -- in various limits -- like the growing number of asynchronous, event-driven networking frameworks implemented in conventional four-dimensional space-time.
Update: There are now 14 projects registered as belonging to TX on Launchpad :-)