Anthony A Barrett raises some interesing points in his excellent Agrippina, Sex, Power and Politics in the Early Empire. Since Britannicus was epileptic, the darkening of his body after death indicated death by tetanoid epilepsy. He claims that no known poison except strychnine darkens the face after death, and it was not apparently available then.

Given the fact that Britannicus' epileptic episode at dinner produced no alarmed reaction from his sister, Octavia or his stepmother Agrippina (despite Tacitus' attempts to divine their reactions) tends to indicate that this happened often.

However, the political repercussions of Britannicus' death cannot be underestimated, whether or not his death was by natural causes or encouraged by members of his family. Afterwards, Agrippina spent more time cultivating friendships with members of ancient Republican families, and Barret states that "the mere belief that Nero was responsible would by itself create an atmosphere of tension and fear."

All in all, an interesting hypothesis to complement the claims of the early empire paparazzi!