The scarf will have three colors, because that is the closest integer to Pi. I will knit 3 rows of the 1st color, one of the second color, four of the next color, etc, etc - until I have an irrationally long scarf, or I run out of yarn, whichever comes first.
I wanted to make this post today at exactly 9:26:53, but Blogger won't let you be that specific. Oh, it lets you set the time to show the seconds, but you can't actually use them.
Over on Ravelry, I have my Pi Scarf project all Queued up, and plan to start that project at the right time, with the help of time.gov and command-shift-3.
Update: Ravelry Project Link