First lets look at the requirement for panels. 408.16(A) requires that a lighting and applinance branch circuit panel "shall be individually protected on the supply side by not more than two main circuit breakers or two sets of fuses having a combined rating not greater than that of the panelboard". Two devices would only be used for a "split bus" panel. There is nothing in this section or any other code section that requires the panel to have a main breaker. The feeder overcurrent protective device is permitted to provide the required panel overcurrent protection. This means that any time you install a subpanel in the same building as the main panel there is no requirement for a main.
When we get to a second building, 225.31, .32 and .33 require building disconnecting means. 225.33 permits you to use up to 6 breakers to provide the required building disconnecting means. So in a sub panel, if you have six or less breakers, no main is required. This assumes that the feeder breaker has a rating equal to or less than the rating of the subpanel. If you need more than 6 branch circuits in the second building, then you must install a main.
There is no code limit on the number of subpanels that can be served from the main panel. You can also feed a second subpanel from the first subpanel.
[This message has been edited by resqcapt19 (edited 05-16-2003).]