GeekNomad

This will do the trick:

FOR /F "tokens=*" %%G IN ('DIR /B /AD /S *.svn*') DO RMDIR /S /Q %%G

If you're thinking WTF?:
DIR /B /AD /S *.svn*
lists all files that are named ".svn"
/B makes the output "bare" with nothing but the file name
/AD only lists directories
/S recurses subdirectories to include their contents if they match the listing criteria


RMDIR /S /Q [path/name]
deletes the directory [path/dir] and all of it's children

FOR /F processes each item (in this case directories) in the set IN ('[command]') by executing the DO [command]

%%G is a parameter, which in this example is a directory name
"tokens=*" says that all the characters output in a line (i.e. by the dir command) are assigned to the parameter %%G

See the Microsoft Command Line Reference for more on FOR: