What is the author's theme and purpose in "Because I Could Not Stop for Death"?

I see you now have many answers, but since I've written it, I'll go ahead and add mine anyway...

The primary theme in "Because I Could Not Stop for Death" is death and eternity. A secondary theme is unpreparedness. This poem is a salute to the inexorability of death, to the dogged journey it traverses. There are more than one opinion as to whether Dickinson's poem speaks of Christian consolation or not. Some view it as devoid of religious or any other consolation and see it as an expose, as it were, on the continual presence of the companion Death. Such an understanding would have been unpopular in the end of the nineteenth century even though religious conformity had lost its hold on spiritual thought.

Dickinson illuminates the primary theme of death and eternity, of the inexorability of death, by placing Immortality as a passenger in the carriage and by describing the centuries of Death's journey as "shorter than a day." The theme of unpreparedness is illuminated by, for example, the "Gossamer" gown with "Tulle" "Tippet," which is a fine see-through silken gown with loosely woven silk netting for a shawl.