Sometimes you\'re developing and you decide to commit, forgetting you created a few files on your project. Then a few days down the line your buddy gets your build out of Su
If you're running on Windows, you could do something similar to Greg Hewgill's answer using PowerShell.
(svn stat) -match '^\?'
This could be extended pretty easily to find all unversioned and ignored files and delete them.
(svn stat "--no-ignore") -match '^[I?]' -replace '^.\s+','' | rm
Hope that's helpful to someone!