I realize this is a few years old, but when i bumped into it I saw that noone actually answers completely answers the question ...so I thought I would post a full solution.
Soooo...
Simply put:
SELECT RoleId, RoleName FROM aspnet_Roles;
GO
But for getting RoleIds for a User it is like this:
SELECT UR.RoleID, R.RoleName FROM
aspnet_Users U, aspnet_Roles R, aspnet_UsersInRoles UR
WHERE U.UserName = @Username
AND UR.UserId = U.UserId
AND UR.RoleID = R.RoleId
GO
This will give you a 2 column list of RoleIds and RoleNames for a particular user.
In reality, you should not be trying to do this as there is a potential for Security breach when a RoleId is exposed. You should only work with RoleNames and use the Membership and Roles methods to manage things.
Hope this helps :)