Merriam-Webster's Word of the Day for March 11, 2010 is:

will-o'-the-wisp • \will-uh-thuh-WISP\  • noun
1 : a light that appears at night over marshy ground *2 : a misleading or elusive goal or hope

Example sentence:
Though her friends think she's chasing a will-o'-the-wisp, Alexis is determined to quit her job and follow her dream of becoming a pop music star.

Did you know?
The will-o'-the-wisp is a flame-like phosphorescence caused by gases from decaying plants in marshy areas. In olden days, it was personified as "Will with the wisp," a sprite who carried a fleeting "wisp" of light. Foolish travelers were said to try to follow the light and were then led astray into the marsh. (An 18th-century fairy tale described Will as one "who bears the wispy fire to trail the swains among the mire.") The light was first known, and still also is, as "Ignis Fatuus," which in Latin means "foolish fire." Eventually, the name "will-o'-the-wisp" was extended to any impractical or unattainable goal.

*Indicates the sense illustrated in the example sentence.