It is unlocked by the guy in the front of the house.He is only there when the quest "Holy Water" is active and you can get the quest from Bigsley if you didn't blow up Megaton.*Note that there is nothing unique inside and there no an achievement or trophy for completing this quest. You will also need the Broken Steel add-on installed.  I hope this helps! xD
X-Box 360- Recieved the Quest to help Scribe Bigsley with Aqua Pura delivery to Megaton (must have completed the Waters of Life quest). Upon fast travel to Megaton, (if you play good character) the lady that brings you free stuff shows you a copy of a note about some "crazy cult in Springvale" that stole all of the water from Megaton. If you go to "My House" you will also find the same note tacked to the front door. After fast traveling to Springvale, you see a guy; "Brother Gerard" who gives you a bottle of water (Holy Water in your inventory) and refuses to speak to you until drink the water. At this point I simply hid and picked the lock (not knowing until later the holy water was in my inventory). In the basement you find Mother Curies Church. From the dialogue you discover what happened to the water, and have a choice of ventilating the whole group for being crazy and stealing water, or you can reason with "Mother" and she will see the error of tricking people to drink poisoned water in an effort to "enlighten people" which ends peacefully. Either way, at this point you return back to Brother Bigsley at the Jefferson Memorial and tell him the problem of the stolen water has been dealt with.