How do you remove Subversion control for a folder?

Also, if you are using TortoiseSVN, just export to the current working copy location and it will remove the .svn folders and files.

http://tortoisesvn.net/docs/release/TortoiseSVN_en/tsvn-dug-export.html#tsvn-dug-export-unversion

Updated Answer for Subversion 1.7:
In Subversion 1.7 the working copy has been revised extensively. There is only one .svn folder, located in the base of the working copy. If you are using 1.7, then just deleting the .svn folder and its contents is an easy solution (regardless of using TortoiseSVN or command line tools).

Try svn export.

You should be able to do something like this:

svn export /path/to/old/working/copy /path/to/plain/code

And then just delete the old working copy.

TortoiseSVN also has an export feature, which behaves the same way.

On Linux, this will work:

  find . -iname ".svn" -print0 | xargs -0 rm -r