Tileing over green board has been done for years. Altough not recommended to much anymore because of all the other better products on the market today.
Some better quality tile wholesalers sell a paint that can be put on the wall that makes it impervious to moisture intrusion. However you still need to deal with the edges which is where all the issues start. And use the recommended thinset material sold by the company that sells the sealant.
Using cement backer board is also an option although a little more difficult to work with you can always use it along the bottom edge of the wall where it meets the tub to prevent any moisture from wicking up into the wall and causing damage. BE sure to use a good quality silicone sealant on bottom gap, BEFORE you tile over the material.
Still another way. Which is how we do our baths.
We use and Densshield sheetrock sold at the local big box store by us. This stuff does not allow mold to develop and from past experiance does not decay if it gets wet. Its surface is fiberglass not paper. Once installed we use Kerdi a sheet material sold at your better tile stores and manufactured by Schluter
This is a felt type material with a mosture and vapor barrier material sandwiched in between.
http://www.schluter.com/ Once properly applied onto the wall and the tile installed on top you will never have to worry about moisture again.Its a little pricey but the cost of rebuilding a wall when it fails is even more.
But once again its very important to properly seal the space between the tub and the wall. That is where all the issues begin. If you think about it. When have you ever seen tile fail in the middle of the wall because of water issue, unless the grout failed. It all starts by the wicking action along the tub or shower base edge. Good luck with the project and have fun!
