See: Multiple Sign-in for Gmail and Google Accounts
And if you have never signed-up for YouTube before, you can now create a new YouTube account with your existing Google Account. [you'll still need to supply an additional YouTube username though which is kind of strange]
Everything remains the same except that a new YouTube icon will show up in your Google Account Services page.
What's a Google Account - if you are on GMail, Google Groups, Adsense, iGoogle, Google checkout, etc - that's your Google Account - it may not necessarily be a @gmail.com address.
If you would like to unlink your YouTube and Google Account association, goto youtube.com/my_account_unlink.
If you unlink your accounts, you will no longer be able to login to YouTube with your Google account. Your YouTube account will stay the same otherwise, and you can still login with your YouTube username and password.
See: Secure your Google and Gmail account