Arne Brachhold created a nifty tool to generate a sitemap for WordPress blogs compliant with Google Sitemaps specifications. His latest release is compatible with WordPress 2.0.
Upload sitemap.php via FTP to your Plugins folder and activate it at the admin panel. Click on Options > Sitemap to edit options and/or generate the sitemap. Everytime you publish a new post, the sitemap is instantly updated.
Login to your Google Sitemaps and submit the new sitemap.