Does a woman's G-spot actually exist?
The existence of the G-spot remains highly debated among researchers.  Two recent publications (Hines 2001, Kilchevsky et al. 2012) review studies of the so-called G-spot and conclude the evidence is too inconsistent to claim its definite existence.  Foldes and Buisson (2009) argue that the G-spot is actually just a clitoris that is richly  innervated. This would be consistent with the reported variability of G-spot presence or absence due to natural variation in innervation among women. (This last statement is my hypothesis offered without citation).

As noted by @Armatus in the comments, a few minutes following some of the Wikipedia links to the scientific journals (not the BBC, CNN, pop news sites, etc.) would give you lots of information about the G-spot. Still, I thought you question was worth typing up a brief answer for the sake of completeness.

Literature Cited

Foldès, P. and O. Buisson. 2009. The clitoral complex: a dynamic sonographic study. The Journal of Sexual Medicine 6: 1223–31.

Hines T. 2001. The G-Spot: A modern gynecologic myth. American Journal of Obstetrics and Gynecology 185: 359–62.

Kilchevsky, A, et al. 2012. Is the Female G-Spot Truly a Distinct Anatomic Entity? Journal of Sexual Medicine. 719-726.