How do I set the creation time of a file?

touch -t YYYYMMDDhhmm file.ext

This will set the modification and creation dates of a file. I ran it on a file I created this morning, and here's what Get Info says:

SetFile can also be used to set the creation date before the modification date or in the future.

-d date    Sets the creation date, where date is a string of the
           form: "mm/dd/[yy]yy [hh:mm:[:ss] [AM | PM]]" Notes:
           Enclose the string in quotation marks if it contains spa-
           ces. The date must be in the Unix epoch, that is, between
           1/1/1970 and 1/18/2038. If the year is provided as a two-
           digit year, it is assumed to be in the 21st century and
           must be from 00 (2000) through 38 (2038).

This would set the creation date to the modification date:

SetFile -d "$(GetFileInfo -m test.txt)" test.txt

SetFile and GetFileInfo are part of the command line tools package, which can be downloaded from developer.apple.com/downloads or from Xcode's preferences after installing it from the App Store.