Is it possible to watch cable TV (specifically Comcast in Massachusetts) through the monitor of a computer running Ubuntu? If so, what additional hardware and software are required for this? Better yet, is there a step-by-step how-to for something like this?
You will also need some software. I like MythTV, and it has a list of digital tuners that work with MythTV.
There are also analog tuners, but you will probably need a digital tuner unless you have a digital to analog converter box. The signal may be encrypted, so you may need to use the box and get an analog tuner.
Here is a page with links to different supported cards.
I know nothing about ubuntu but the hardware you need is called a TV tuner card. Actually I think they make external ones too. Make sure it accepts the cable input (coax or whatever). The software will then allow you to watch TV in a window and you can continue working with the OS. You can also record. I found this via google, maybe this app is the right one to use with your hardware: http://ubuntuforums.org/showthread.php?t=230279
Without knowing what your specific cable setup is like, here's some broad advice:
If you don't have a set top box, the kind of tuner card that CreeDorofl suggests will grab unencrypted analog content. The number of analog channels available without a box is shrinking over time, there probably isn't much available except your local channels.
If you do have a set top box, something like this will grab unencrypted digital content (ClearQAM) as well as broadcast ATSC. What unencrypted content is present, if any, will vary from provider to provider. It sounds like Comcast doesn't have much nowadays.
Sounds like Comcast does CableCARDs, but requires a truck roll.
For Comcast, it seems like there won't be much content available without a CableCARD.
FYI, I'm sure there are similar products on the market, the Silicon Dust product line is just what I'm familiar with.
Getting a signal up on your display so you can watch it live is a start, but you'll probably want to be able to record it and watch it later. You're going to want MythTV as tjameson suggests. Here are some instructions for getting a Silcon Dust receiver working with MythTV on Ubuntu.