It is not always necessary to postulate an adaptationist explanation for every phenomenon of human nature, even though it may be very useful for some. It is entirely possible that art is an entirely cultural innovation which requires no evolutionary explanation. At the very least, art of the written form (poetry and form) almost definitely has no evolutionary basis, since no significant evolutionary change could have occured since the inception of writing. Visual arts are a more blurry area, as they date back significantly farther. However, one might postulate that cave paintings and the like served a role in pre-linguistic communication. I'm not particularly familiar with human cultural evolution (I'm taking a class on human evolution next month and intend on reading Jared Diamond's book as soon as I finish the latest Dennett), so my guesses on the matter might be innaccurate.
However, one art form which definitely requires an evolutionary explanation is music, since humans display an innate propensity for it almost the equal of our propensity for learning language. A variety of studies have shown that infants are as accurate in recognizing pitch and melody as adults (except for those with formal musical training.) As to the question of how (or why) music evolved, the sexual selection issue probably played a significant role. Other ideas which have been proposed include the use of music as a sort of cultural identifier (allowing for sorts of early tribal "patriotism".) I remember there was an interesting feature on PBS dealing with this topic recently, if you can get a chance to locate it, it was well worth the hour spent watching it.
Anyway, just my two cents.