Is it common for the CEO to be a member of its board of directors?

I can only answer this from the standpoint of a non-profit. If you're talking about a for-profit corporation, this does not apply.

The easy answer is "sometimes." In my state (Pennsylvania) it's generally accepted that your CEO should NOT be a voting member of the Board of Directors, as it could potentially cause a conflict of interest (The Board sets Executive Salary. The CEO reports to the Board of Directors). That being said, the CEO should attend all Board meetings of a non-profit to provide a report on the status of the organization.