I recently tackled a problem like this while making Chat Mapper. What I do is graphically plot out the dialogues as nodes in a tree and then each node has a condition and a script associated with them. As you traverse through the tree and hit a node, you check the condition to see whether or not that node is valid, and if it is, you execute the script associated with that node. It's a fairly simple idea but seems to work well from our testing. We are using a .NET Lua interpreter for the scripts.
