My Uncommon Slice of Suburbia, has an amazing step-by-step guide for converting solid wood doors to glass front. It actually looks a lot easier than I originally expected! Although, I admit, I think this project would only work with painted doors, as I'm sure there will be a lot of scuffing and chipping that will require touch-up.
Kristin's tutorial makes it seem pretty darn easy! Simply cut the middle panel out of the door, add trim to cover the rough cuts, insert glass (cut to size at a hardware store or glass shop), and, lastly, glue the glass into place! After letting the glue dry overnight, you can add mirror clips or trim to the back of the door to give the glass a little added security. Kristin mentions it on her blog and I'll repeat it here: this project is not reversible. Once you've cut your door, there's no going back so you're going to be married to this look for a long time coming! Check out the full tutorial on Kristin's blog: My Uncommon Slice of Suburbia: Adding Glass to Your Kitchen Cabinets. Images: My Uncommon Slice of Suburbia