Well, the new (yet to be released) Tivo2 (The *Next* Generation), will supposedly have a place where a second Tuner board can be connected.

If they build this feature into the new models, then yes, you will be able to probably record two programs and watch a third.

However, whether or not both Tuners are regular 'over the air' tuners is something I don't know [as opposed to one being a Dish/Satellite tuner].

Two Over the air tuners would require 2 video encoders - 1 for each Analogue video source.

I don't know the hardware specs on the new Tivos except that they are not PowerPc based anymore (unlike the original Tivo which was/is) and use different processors and [presumably] different encoder/decoder and would require different software from the current Tivo SA units - hence the probability of a PiP feature esp. as the new Tivo2 remotes have a button labelled for this purpose.
And of course the new Tivos come with 2 USB 'master' ports 'for future expansion' a bit like the HSX-109 does - like Ethernet and Keyboard etc.