It's not very difficult to do. You'll have to spend a few bucks, but it works like a charm. I have an HD Tivo, and an SD Tivo, the process is slightly different depending on what you have, but uses the same equipment.
First, you'll need to pickup a Dazzle Holywood DV Bridge, should be able to find it at your local computer store or online - price that I saw was around $300.00 - but you may be able to find it cheaper. May seem like a bit of money to invest, but trust me, it's worth it.
Second, you'll need iMovie, which should have come installed if you have a fairly new Mac. If not, you can get iLife '06 for under $100, which includes all the "i" applications.
Once you have those two items, you're good to go. You'll have some options as to how you want to connect your Tivo to the DV Bridge - S-Video or Composite Video, plus your Red and White audio cables. You'll go OUT from your Tivo, to the IN on the DV Bridge - then from the DV Bridge OUT, you can either use a FireWire 400 connection (which is what I use) or if you have a capable A/V card you can run your A/V cables out from the DV Bridge directly to the inputs on your computer. I personally recommend the FireWire 400 connection, one cable does it all.
Once you have all that setup, launch iMovie. It'll ask you to create a new project. After you've created a new project, it'll take you to the main screen. You may get a message that says your capture device can't be found - don't worry about this, it even says in the message that with some equipment it may not be found - but you're fine, just click ok.
Now, if you have an SD Tivo, you should be good to go. If you have an HD Tivo, then you'll have to change the resolution on your Tivo box to 480i. Otherwise you won't be able to capture your video.
If all is set correctly, whatever is currently set on your Tivo box should be playing on the iMovie screen. If you want to capture what's currently playing, just click the Import button and it will start capturing. If you want to capture something you've saved on Tivo, go into your Now Playing list, select what you want - and I normally do the Save to VCR, that gives me a couple seconds to start Importing in iMovie, otherwise you'll click play and have to click Import pretty quickly.
Hopefully that will get you going - let me know if you have any questions or run into any snags.
HAPPY CAPTURING!