A few days ago hacker Chpwn, who ported Siri over to the iPhone 4 and iPod touch 4G, teamed up with another hacker (Aman Gupta) to find a way to make Siri accept custom commands. With both Chpwn and Aman Gupta vigorously working on the challenge we are glad to tell you that they indeed succeeded getting Siri to accept custom voice commands.
In the picture below you can see Siri accepting a custom command.
Although they finally managed to get Siri to accept custom commands, it is a very difficult and complex procedure. Additionally, the method they used to hack Siri is surely something Apple will not just pass by if released to the public. Thus, they will not make a public release of a how to make Siri accept custom commands guide, but we hope they will soon figure out a legal and safe way of doing just this in the near future.
Keep an eye on iJailbreak.com, and you will be the first to know of such a guide when it is released to the public.