How to change the creation (st_birthtime) date/time of a file in Lion?
touch -t also changes the creation time if the target modification time is before the original creation time.

SetFile can set the creation time to be before the modification time 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 time to the modification time:

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


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