You'd be able to tune those channels without a cable-card, but my understanding is that without the cable card to provide the channel mapping the TiVo doesn't know that which clear-QAM channel maps to (for example) "CBS". And it doesn't have any manual remapping functionality; so you can't tell it that yourself.
If the TiVo can't map the channels it won't know what guide data goes with each channel and things like season passes and wishlists won't work. I believe you'd still be able to schedule manual recordings by time and channel; but most of the usefulness of the TiVo relies on accurate guide data/channel mapping.
Xbox: MetalThreshkeen -- PSN: Threshkeen