There are many variations of tree edit distance. If you can go with top-down tree edit distance, which limits insertions and deletes to the leaves, I suggest trying the following paper: http://portal.acm.org/citation.cfm?id=671669&dl=GUIDE&coll=GUIDE&CFID=68408627&CFTOKEN=54202965. The implementation is a straightforward dynamic programming matrix with O(n2) cost.
