The Crowd plugin should support Single-Sign On. This means:

• If the crowd.token_key cookie is set, authenticate using the token.

And one of:

• When login is required, present the standard Hudson login form and set the
  crowd.token_key cookie.
  OR
• When login is required, redirect to the Crowd login form.

Even better would be to support both patterns and allow the admin to select one
or the other.