Colin had a good post over at NewMusicBox this week about giving pieces titles (an unusually fine comment thread, as well). He takes Penderecki to task for his opportunism* in renaming the originally abstract "Threnody" to be a remembrance of the victims of Hiroshima, but concedes that it was a canny move: "I'd wager that the ugliest, most ear-splitting piece imaginable could win over a 21st century crowd if it had the perfect title."
Here's the problem/opportunity with titles on pieces of music: once you move outside the most bland and academic appellation—either a shout-out to the basic form (Sonata, Suite, etc.) or a list of who's playing (String Quartet, Symphony)—you're essentially using the language in a surreal way, and that sometimes results in a bit of cognitive dissonance with the piece itself. Describing music is a notoriously difficult challenge for language, and to sum up even the simplest piece in less than ten words is pretty much impossible. Which means any title you apply is a) somewhat disconnected from the piece, and, more interestingly, b) somewhat disconnected from itself: you're taking words that normally have agreed-upon meanings and putting them in a situation where either they become meaningless by virtue of their necessary inadequacy, or they're being used in a deliberately misleading way. Both results are traditionally surreal: the language is being alienated from itself, either directly or indirectly.
If your aesthetic is a surreal one, this isn't a problem; you can easily come up with titles that not only aren't jarringly incongruent with the music, but actually contribute to the overall effect. (Think of Satie.) But if your artistic goals are less informed by the powers and pleasures of absurdity, titles can be a tricky business. The safe way to go is the previously mentioned bland and academic path, but of course, you're surrounded by people telling you that you need to do more to get people interested in your music, that you need to grab their attention and get them to want to hear what you've written. They've got a point: the marketing possibilities of an effective title are not inconsiderable, and few of us have trust funds that would enable a willful ignorance of the realities of the marketplace. But if you're not referencing an artistic tradition (like Surrealism) where a certain amount of bait-and-switch is not only expected, but welcomed, you run the risk of disappointing the guileless and annoying the skeptical: if you call your offering "Elegy to 9/11," plenty of people will find it to be a shallow experience compared with their expectations, and plenty of others will find it to be nothing more than a cheap stunt.
What can you do? You can set texts: naming the piece after the poem that's being sung shifts the above burdens to the poet. (This can work for non-vocal music as well: find a pre-existing literary title or quotation that has a vague connection to your piece, and it has a certain inoculating effect. Boulez does this all the time; I've resorted to it on occasion.) You can take refuge in a certain hip snarkiness: the Bang On a Can types do this a lot, and often quite well, coming up with titles that are just abrasive and anti-establishment enough to give the expectation of listening that frisson of sitting at the cool table in the cafeteria. You can opt for puns: titles become such obvious and deliberate jokes that they detach themselves from the piece, and become more of an intellectual amuse-bouche for the listener. (Milton Babbitt and Michael Gandolfi are the acknowledged masters of the practice.) You can reference events or relationships so private that the audience is completely locked out of determining the appropriateness of your choice. (Think of every piece you've ever heard with a title like "For [person known only to the composer].") Or you could go ahead and name-drop a great event/historical figure/tragedy, and hope that you get away with it. (It worked for Penderecki, in spades.)
I guess I'm lucky in that my own music seems to lend itself to a certain amount of leeway in creative titling, but that's probably because my musical taste has also been heavily influenced by my literary taste, which does run towards the surreal. I like linguistically alienating effects; I like focusing on the meanings of individual words out of context; I like the poetic point where one can slip back and forth between the sounds of words and the meanings of words a little too easily. So it's only natural that I also like playing in the space between a piece of music and its title. But I'll admit that the ground there gets a little slippery at times.
*Update (12/17): as Alex Ross rightly points out in the comments, how responsible we should hold Penderecki for the renaming is unclear given the murky moral ground of totalitarian Poland.