No, the USB ports are only for a wireless adapter.
Current as in literally available today, I presume you're correct.The current crop of USB tuners are only ATSC and QAM so they will only get you OTA HD programming anyway. Your series 2 Tivo is not designed to handle HD content so it wouldn't be feasible even if the USB ports supported the tuner. The Tivo itself has no means of utilizing an outboard tuner because of the way it's designed.
The Ceton USB tuner is not yet available and no date has been set for its introduction. SiliconDust just started shipping their 6-tuner version of the HDHomeRun Prime networked tuner and the 3-tuner version is expected to follow shortly. All cablecard tuners are currently certified to work only with Windows 7 Media Center. Hauppauge has announced a dual tuner model, but it's internal only.Current as in literally available today, I presume you're correct.
But aren't there USB versions of the Ceton or similar 'cards' on the way? (As in done, taking orders, but not quite shipping yet?)