Sitemap plugin: mkdir() error on Windows IIS and fix

I have installed WPMU and Windows IIS, I usually don't like saying this too loud because I get all the purist Linux folks booing. But then again, I might not be alone using a Windows plateform, so I thought I'd post this in case it helps someone out there.
The "Sitemap and SEO" plugin produced the PHP warning "PHP Warning: mkdir() [function.mkdir]: Invalid argument ... in simple-sitemaps.php on line 101".

I have changed slightly the syntax for the function writefile() in simple-sitemaps.php to the following:
function writefile( $filename, $content ) {
$parts = explode( '/', $filename );
$file = array_pop( $parts );
$filename = '';
foreach ( $parts as $part ) {
if ( !is_dir( $filename.=$part.'/' ) ) mkdir($filename);
file_put_contents( "$filename/$file", $content );

This fixes the problem (for IIS7 and PHP5).