I went the PC route and combined several pieces of equipment into one.
Not bragging, just explaining:
AMD Phenom X4 9850 quad core processor and 16gigs of ram with an 80g system drive and two 1tb hard drives combined as /opt in an LVM (which will grow to 4tb soon.) My set up uses the hauppauge 1600 card which both grabs over the air TV (and I've read can do the digital over the air broadcasts) and does CATV direct. My system of choice is 64 bit Ubuntu with MythTV. It also uses an NVIDIA 9500GT card with a gig of onboard ram and two outs, one to the 42" plasma and one to a 17" lcd for system monitoring/normal computing tasks.
All in one I have DVR/CATV plus music/internet radio/internet tv (if anyone wants the link to getting 64 bit flash working native on their linux box, PM me)/picture viewer etc. you get with something like the apple TV. I also have ripped all my DVD collection in at 1:1 and can watch those from the hard drive. MythTV backend is also serving to the other PC in the house and my laptop for all of the above over gig ethernet/wifi. For a neat trick I installed the MythTV front end on my samsung Q1 Ultra and can watch TV/movies/etc streamed over the wifi to any room.
My set up is NOT for beginners. It took lots of tweaking and command line geekery to bring to working condition. I should point out, though, that the entire cost of putting this server together in terms of money was less than many plasma TV's standalone systems. I already owned the plasma TV however.
Oh, this box also does folding@home and serves HTTP/HTTPS, MySQL, IMAPS, SMTP, SSH and is my network firewall/DHCP server. The internet connection for my home network goes directly to this box with an onboard ADSL modem from Sangoma and a gig NIC going to a gig switch. Wifi is done via an onboard wireless G card and is firewalled off from the wired network.
All of this put together was less than 600 bucks (not including parts I already had, like the wireless card, the ADSL Sangoma modem and the TV.) I bought all the components in January at MicroCenter, so the sales deals were killer. Cost in time was .... well who am I kidding, I never stop tweaking this thing (It's the geek in me.) With the right time input, you can do quite a lot with MythTV and some relatively cheap hardware.
One note: The Hauppauge 1600 is a BEAR to get working under linux right now. I suggest going with another card unless you want to either A) put a monstrous amount of work into getting it working flawlessly or B) buy it and wait another 6 months for full kernel support. Trust me on this.
Even if you just get an intel mac mini and the EyeTV adapter (this is what I used to run) it's a better investment I think to go the computer route as you can then do very flexible things compared to the standalone stuff.
In the end, I consolidated 14 seperate power plugs down to 2.
ETA: Forgot to add:
1) I only have the barest possible cable package, so there's no set top box. the coax just hooks up to my computer. YMMV, some people like having more channels.
2) Windows/Linux/Mac: You can use the firewire output on your set top cable box to directly input to your computer. It's tricky, but worth it. Google around for instructions. For MythTV the matrix on firewire compatibility is here:
http://www.mythtv.org/wiki/Firewire_Cable_Box_Compatibility