Over time, it is definitely possible for regular consumption of lemon juice to erode part of the outside enamel layer of our teeth. Lemon juice is highly acidic, and it's especially high in one acid called citric acid.
The enamel surface of our teeth is highly mineralized; the key mineral in our tooth enamel is called hydroxyapatite, and it's primarily a calcium- and phosphorus-containing crystal-like substance. The chemistry of citric acid and calcium is such that certain forms of each substance like to join together and form calcium citrate. This electrochemical fit between citric acid and calcium is the reason that fruits high in citric acid can erode the tooth enamel. Most of the studies on this subject have involved orange juice rather than lemon juice. The results with orange juice are sometimes mixed, but orange juice (with a pH between 3 and 4) is not as acidic as lemon juice (with a pH between 2 and 3). Still, for both citrus fruits, the principles behind tooth enamel erosion would be basically the same.
Some healthcare practitioners suggest drinking citrus fruits through a straw to avoid this possible damage to the tooth enamel. I don't have any problem with that suggestion. But what I would like to see (and have not seen) is research showing exactly how much lemon juice it takes to produce this possible erosion of the tooth enamel. I'd guess that a very small amount (for example, one-quarter of a teaspoon) in a full eight-ounce glass of water might not have enough acidity to pose a significant risk. Until I see that kind of research, however, I'd recommend any strong lemon juice consumption be done through a straw and that lemon water be made as dilute as possible, with just enough lemon juice to provide a delicious flavor but not enough to greatly upset the naturally neutral pH of pure water (7.0).
For more iniformation on this topic, see:
Barbour ME, Parker DM, et al. Human Enamel Erosion in Constant Composition Citric Acid Solutions as a Function of Degree of Saturation with Respect to Hydroxyapatite. J. Oral Rehabil. 2005(1):16-21.
Rees J, Loyn T, and Gilmour A. Does Low Acid Orange Juice Equal Low Erosion? Dent Update. 2006;33(4):242-4.
West NX, Hughes JA, et al. Development of Low Lrosive Carbonated Fruit Drinks 2. Evaluation of an Experimental Carbonated Blackcurrant Drink Compared to a Conventional Carbonated Drink. J Dent. 2003;31(5):361-5.
Hunter ML, Hughes JA, Parker DM, et al. Development of Low Erosive Carbonated Fruit Drinks. 1. Evaluation of Two Experimental Orange Drinks in Vitro and in Situ. J Dent. 2003;31(4):253-60.
Oussama A, Touhami M, Mbarki M. In Vitro and in Vivo Study of Effect of Lemon Juice on Urinary Lithogenesis. Arch Esp Urol . 2005;58(10):1087-92.
Seltzer MA, Low RK, McDonald M, et al. Dietary Manipulation With Lemonade to Treat Hypocitraturic Calcium Nephrolithiasis. J Urol. 15 1996;6(3):907-9.