What is the de facto forensics Linux distro to use?

Check out Backtrack : http://www.backtrack-linux.org/

There isn't one, though the SIFT workstation would probably be closest for law enforcement since it's put out by SANS (who trains law enforcement).  The vast majority of American LE uses Encase/FTK/both.  Helix, Raptor, and Caine would probably be among the most popular live CD distros.    I've always considered Backtrack to be heavier on the network intrusion side of things rather than traditional filesystem digital forensics. The distro itself isn't really relevant, they just provide the convenience of having many potentially useful tools in one place.  The tools you're using matter to an extent, but more important is how you use them, test your findings, and produce evidence that's verifiable.  Just because you use EnCase, FTK or the Linux distro du jour doesn't mean your findings won't be blown up in court.

When running a forensic investigation against a machine that you know will be reviewed in the court of law, you want to use a tool that has a proven track record and has been vetted by the legal system. Using unproven/uncommon tools, while useful, may bring up the question of whether the said tool is trustworthy/reliable and can potentially cause problems. For this purpose (and others) a lot of organizations/LEs use EnCase or FTK. Other than that, there's no single too for this type of job. Most people recognize sleuthkit as a good goto solution: http://www.sleuthkit.org/. For memory analysis, lots of people trust volatility (http://code.google.com/p/volatility/).

Helix and deft are the best Linux distributions to use for forensics.

In addition to Backtrack, Paladin and Deft are also very useful Linux distributions.