I've been wanting to jump into the Mac scene for awhile now, and these new iMacs are exactly the excuse I've been needing to force myself to do it. An iMac with a dual-core Intel chip? Man, two years ago that would have been unthinkable.
I think the TiVo is a better option for a few reasons. #1) Noise. #2) Ease of use. Anyone can figure out what to do with a TiVo in 2 minutes. All of my PC based DVR experiments have been horrible failures.