<p>Using subversion 1.5 I have branch B which was branched off of branch A. After doing work in both branches I go to merge changes from A into B (using <code>svn merge http://path/to/A</code> in the working directory of B) and get <code>svn: Target path does not exist</code>. What does this mean?</p>

It means that there was a change in a file in branch A which Subversion tried to merge into branch B, but the file does not exist in branch B, so it has nowhere to merge the change. For example:
Create branch B
Change A/README
Delete B/README
Merge A -> B
Now the change in README cannot be applied in branch B, because the file has been removed there.
EDIT: This will result in a
tree conflict
in Subversion 1.6.