Is this happening to you?

jah$ svn co http://mydomaininc.com/svn/myrepos/
Authentication realm: <http://mydomaininc.com:80> mydomaininc.com
Username: jason
Password for 'jason':
A myrepos/trunk
A myrepos/trunk/sql
A myrepos/trunk/sql/f_import.sql
A myrepos/trunk/sql/r_import_large.sql
A myrepos/trunk/sql/util.sql
A myrepos/trunk/sql/indexes.sql
Checked out revision 393.

Jason-Hoffmans-Computer:[mydomain/trunk/]:jah$ svn diff -r 384:HEAD
svn: GET of '/svn/myrepos/!svn/bc/384/trunk/sql/f_import.sql': 301 Moved Permanently (http://mydomaininc.com)

301 errors happen in two situations:

1. You've changed your DocumentRoot to be the same as your SVNPath or SVNParentPath (Subversion Book FAQs)
2. You have a .svn (dot svn) directory in public_html or public

textdrive-server# ls -l /home/youruser/public_html/.svn
total 18
 r--r--r-- 1 youruser youruser 118 May 7 20:19 README.txt
 r--r--r-- 1 youruser youruser 75 May 7 20:19 dir-wcprops
 r--r--r-- 1 youruser youruser 0 May 7 20:19 empty-file
 r--r--r-- 1 youruser youruser 1040 May 17 16:34 entries
 r--r--r-- 1 youruser youruser 2 May 7 20:19 format
drwxr-xr-x 2 youruser youruser 512 May 7 20:19 prop-base
drwxr-xr-x 2 youruser youruser 512 May 7 20:19 props
drwxr-xr-x 2 youruser youruser 512 May 17 16:34 text-base
drwxr-xr-x 6 youruser youruser 512 May 17 16:34 tmp
drwxr-xr-x 2 youruser youruser 512 May 17 16:34 wcprops

textdrive-server# mv /home/youruser/public_html/.svn /home/youruser/public_html/usedtobedotsvnbutjasonmovedit

Jason-Hoffmans-Computer:jah$ svn diff -r 384:HEAD
Index: FileThatIsUnderSVN.txt
=====================================
--- FileThatIsUnderSVN.txt (revision 384)
+++ FileThatIsUnderSVN.txt (revision 393)

Moral of the story?

1. DocumentRoot and SVNParentPaths can't be the same and
2. A dot.svn folder in your DocumentRoot will prevent diff, merges etc.