One of Gmail's greatest features is its spam filtering. But, if you have a situation where you want to forward mail from multiple Gmail accounts to a single location, that fantastic filtering becomes a little cumbersome. In this situation, having junk mail sitting around in multiple Gmail accounts is actually the opposite of what you want. It would be better for all mail, including spam, to be forwarded so that you can waste time digging through a single spam box rather than many.
The idea is to make sure all email is forwarded even if it is spam. Dustin's trick is to forward all email which doesn't contain the EICAR antivirus test string. Since Google blocks mail with this string, it should guarantee to match all mail and it has the advantage of matching mail when a From: or To: header is not included.
Here's the EICAR antivirus test string: