The email you receive from youtube says (with subject: Subscription Update) :
Your Latest Subscription Videos – August 26, 2011
Here are the newest videos from each of your subscriptions. To change the subscriptions for which you receive updates, or to turn off updates, go to your email options.
To change the settings, simply click the link saying “your email options” and it will take you to your youtube account where it will give you options to change your youtube email settings.
How to disable youtube subscription event:
To completely stop youtube subscription events select the option: “Don’t send any emails for the above events.”
How to stop youtube weekly subscription emails:
To disable weely update emails from youtube simply uncheck the option saying: “Send subscriptions updates once each week”
You can also change youtube newsletter settings by checking or unchecking to receive it.
Save the settings to let them take affect.