Theoretically: could Obama be appointed to cabinet?

[Hnv1]

Well on the simple reading of the constitution the answer is yes. But that will put him in the presidential line of succession thus potentially giving him more time in office.

[Hnv1]

Of course, though it would be unclear whether he could be in line for presidential succession. That question would be very theoretical, though, since it's not likely anyone beyond the speaker would ever become acting president.

I believe I read somewhere the Kennedys actually intended to have Jack as Secretary of State under President Bobby Kennedy once he finished his second term in 1969 and Bobby succeeded him.

well, technically he can run to congress and be elected as Speaker.