I forget where I found this, but it's been useful to me:

Error:
"svn: Can't move '.svn/tmp/entries' to '.svn/entries': Operation not permitted" (in Terminal, when trying to check-in or update the repository)

Fix:
Use 'chflags -R nouchg .' on the command line. Note '.' at the end. It is not optional.

Here is what the options do:

-R: "Change the file flags for the file hierarchies rooted in the files instead of just the files themselves."

'nouchg' is the negated form of: "uchg, uchange, uimmutable set the user immutable flag (owner or super-user only)"