I found a commandline util called xattr which let me read/change/delete them and found that the m3u file (downloaded) had an attrib named com.apple.FinderInfo which it seems that release of itunes would refuse to allow as a valid file for import.
Calling xattr to delete the attrib before the import fixed it. It’s now no longer required.

Interesting/frustrating/random or what!?

H
Here’s my whole m3u-to-iTunes script anyway:

on open m3uFileList
tell application “iTunes”
activate
if minimized of browser window 1 then
–display dialog “iTunes was mini”
set minimized of browser window 1 to false
end if
set newPlaylist to make new user playlist
set name of newPlaylist to do shell script “date +%Y-%m-%d\\ %H:%M:%S”
repeat with m3uFile in m3uFileList
–display dialog “trying to load ” & m3uFile & ” from list: ” & m3uFileList
set f to POSIX path of m3uFile
–display dialog “Running xattr on ” & f
–do shell script “/usr/local/bin/xattr -d com.apple.FinderInfo ” & quoted form of f
add m3uFile to newPlaylist
tell application “Finder”
delete file m3uFile
end tell
end repeat
set the view of browser window 1 to newPlaylist
play newPlaylist
set minimized of browser window 1 to true
end tell
end open

http://www.jforsythe.com/jforsytheblog/2008/01/24/ApplescriptITunes76AndFirefox20011M3uFileUnfcker.aspx