As others have mentioned, Miller promulgates a three-part test for obscenity (and which should not be confused with Pacifica’s indecency). Those elements are:
[ol]
[li]the average person, applying contemporary community standards would find that the work, taken as a whole, appeals to the prurient interest,[/li][li]the work depicts or describes, in a patently offensive way, sexual conduct specifically defined by the applicable state law, and[/li][li]the work, taken as a whole, lacks serious literary, artistic, political, or scientific value.[/li][/ol]
Now, in general, none of HBO’s programming satisfies all three prongs of the test. However, the OP does ask a very subtle question, to wit, “what is the community for a nationally broadcast program?”.
In Pope v. Illinois, the SupremeCourt clarified that “the first and second prongs of the Miller test–appeal to prurient interest and patent offensiveness–are issues of fact for the jury to determine applying contemporary community standards.” The third prong, the Court taught, is not determined according to contemporary community standards but whether a reasonable person would find such value in the work. The Court reasoned that serious literary, artistic, political, or scientific value inhered in the work and did not vary from community to community.
The Court has never told us what precisely the community is whose standards will govern in an obscenity finding. The Court has approved jury instructions where the jury is directed to apply “contemporary community standards” without further articulation of what that means. In Reno v. ACLU, the Court observed that “the ‘community standards’ criterion as applied to the Internet means that any communication available to a nation wide audience will be judged by the standards of the community most likely to be offended by the message.” Although the Court’s elaboration on this point suggested that this would be an unconstitutional state of affairs, it did not decide the issue since the CDA was rejected on other grounds (overbreadth). Finally, in Ashcroft v. ACLU (2000), the Court approved of the use of “contemporary community standards” in a federal statute outlawing material on the Internet that was harmful to minors. The argument paralleled the fear in Reno, that such a criterion would mean that material on the Internet would be subjected to the community standards of the most restrictive community. The Court held that like Miller and unlike the CDA, the additional prongs of the test, including the community-invariant “serious value” prong, saved the COPA from overbreadth.
So, it seems that an uneasy balance has been struck and the Court is content to allow “contemporary community standards” to remain ambiguous so long as it is coupled with an increasingly more dispositive “serious value” test. But as to the OP’s specific question, the community whose standards will apply are whatever standards the jury panel understands that phrase to mean.