How can I change a file's creator code on vanilla Snow Leooard?
I ran the following script on a variety of files on my system running 10.7.4. I ran it on files in my home directory, I ran it on files on a mounted FAT16 volume.

I was unable to get it to fail silently. If the file selected was locked or read only, the script failed with the error "Finder got an error: The operation can’t be completed because you don’t have the necessary permission."

Otherwise, in all the cases I tried, the script worked: regardless of the type and creator reported at the beginning, the type and creator reported at the end was TEXT and iDAT. Similarly, when I ran the script again on the same file, it initially reported type and creator of TEXT and iDAT, respectively.

So my advice would be to upgrade to Lion; if the script fails in 10.6, it seems to be a bug that has been fixed in 10.7.

set theFile to choose file
tell application "Finder"
    set myAlert to get file type of theFile
    display alert myAlert
    set myAlert to get creator type of theFile
    display alert myAlert
    set file type of theFile to "TEXT"
    set creator type of theFile to "iDAT"
    set myAlert to get file type of theFile
    display alert myAlert
    set myAlert to get creator type of theFile
    display alert myAlert
end tell