<p>Using the svnmerge.py tool it is possible to merge between branches, up and down.  It is hard to find the details for doing this.  Hopefully, v1.5 will have a neat method for doing this without using svnmerge.py - details requested!</p>

It looks like you're asking about 1.5 merge tracking.  Here's a quick overview for doing merges to/from trunk (or another branch):
http://blog.red-bean.com/sussman/?p=92