While no current Tivo supports PiP using multiple tuners its quite likely that the PiP button on the new remote can also be used to monitor Play-back' and Live-TV at the same time with the current hardware.
[remember the current live-TV signal is ALWAYS being digitised and recorded to disk, so all you need for Live-TV PiP is to decimate & 'overlay' the current digitised video image image into a small window over the currently decoded playback stream - something I think the Tivo can do relatively easily now [or if not now, real soon now].

PiP using live-tv and current program being watch (from now showing) is a feature thats long been 'missing' from the current Tivo software (and often requested in the past) and now that Tivo software version 3.0 is nearer its quite likely that this will be what the PiP button is intended for at least initially.
Once the newer 2 tuner hardware is released then PiP can be used for that like 2 tuner TVs now.

Hmm, I like the idea of being able to watch the live TV last half-hour 'buffer' and in a window have the 'right-now' live-TV signal as a window on the top - that would be cool for live sports - no need to miss the action while you do a action replay of the previous piece of action.

What I would hope is that any PiP window can be moved around the screen, can be resized [from 1/16th to 1/9th, top maybe 1/4 of the screen] and can be swapped with the main picture quickly.

Now this is what a DVR is all about!