I'd heard the news about a month ago that there were some programs floating around that would allow easy decoding of the .tivo format into the .mpg format, but these programs generally were run from the command line on OSX. In doing some searching -- need to clean out some of the stuff on the TiVo to accommodate new seasons of favorite shows -- I ran across the TiVoDecode Manager, which is essentially a GUI interface to the command line application for OSX.
TiVoDecode Manager is very simple to use: simply determine the IP address and media access key from within your TiVo settings, install the software on a OSX machine hooked into the same network as the TiVo, enter the IP address and media key, and TiVoDecode Manager connects to your TiVo and offers a list of download options. The software defaults to downloading and decoding files using MPEG-2 native, which is the same CODEC that TiVo natively uses -- I've been using the MPEG-2 and then batch-converting files to MPEG-4 using VLC because it allows me to manipulate lots of options, but it is important to note that TiVoDecode Manager does include the ability to download as MPEG-4 and even to import MPEG-4 files to iTunes once downloaded and decoded.
As part of m quest to digitive video in anticipation of AppleTV, TiVoDecode Manager is an important piece of software in my arsenal.