For GoDaddy domains this is rather quick and painless to set up. We will start with the DNS settings first and then it is just a matter of making one settings change in Google Apps.
Before we start, if you purchased your domain via Google Apps or Blogger, you may have some extra Google IPs (64.233.179.121, 66.249.81.121, 72.14.207.121 ofr example) floating around in your A records that will give you headaches. Delete them before you start the DNS configuration process.
First create or edit the www CNAME record to point at ghs.google.com by logging into GoDaddy and pulling up the Total DNS settings for the domain you are editing:

Then edit the Forwarding settings for your primary domain to point at your www subdomain:

Now, you may have to wait fifteen to twenty minutes for domain forwarding to activate, but once it has, typing in yourdomain.com in your web browser will automatically redirect to www.yourdomain.com.
To set up Web Pages to show up at your custom domain, log in to Google Apps and browse to your Web Pages settings:

Click on Change URL:

And make sure that your Web Pages URL is set to your custom www subdomain:

If you have followed all these steps, yourdomain.com will redirect to www.yourdomain.com and show your Google Apps Web Pages.