To get a diff of a specific file and a specific revision of that file,
svn diff -r REVISION_NUMBER PATH
To compare a specific revision to another specific revision (instead of your working copy) enter:
svn diff -r REVISION_NUMBER SECOND_REVISION_NUMBER PATH
It definitely helps to do a straight forward grep to see only additions of subtractions:
svn diff blah | grep ^+ & svn diff blah | grep ^-
Yuji Tomita 