Are you talking about a comment within the file, or a commit comment?
If it's the former, then no, you can't go back and modify an already
checked in revision. If it's the latter, then yes, you can change the
log message, as that's an unversioned property.
See:. http://svnbook.red-bean.com/en/1.0/ch05.html#svn-ch-5-sect-1.2
-larry
http://svnbook.red-bean.com/en/1.0/ch05s02.html#svn-ch-5-sect-2.1