I do not know how to do this using pure Subversion. But you can do it with sed :
svn log | sed -n '/username/,/-----$/ p'
This finds each instance of the username, and then prints everything down to the dashed line that marks the end of the log entry.
The search template is very flexible - you can easily change it to search for other things. For example,
svn log | sed -n '/Dec 2009/,/-----$/ p'
Will return all commits made in December (by any author).
Edit: If you don't want to use the actual commit message, but just summary metadata, you can just use grep same way for William Lyra Windows answer :
svn log | grep username
ire_and_curses
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