Using the Finder to do the sort is rather grossly slow - 3 full seconds to sort 55 items on my i7 MacBook Pro with Lion.
on listFolderByModDate(folderAlias)
set posixFolder to POSIX path of folderAlias
set posixFolderQuoted to quoted form of posixFolder
set itemList to (do shell script "ls -tr " & posixFolderQuoted)
set itemList to paragraphs of itemList
return itemList
end listFolderByModDate
on makeAliasList(folderAlias, fileNameList)
set folderAlias to folderAlias as text
repeat with i in fileNameList
set contents of i to (folderAlias & contents of i)
end repeat
return fileNameList
end makeAliasList
set folderAlias to alias "Thor:Users:chris:Pictures:Downloads (New):"
set sortedFileNameList to listFolderByModDate(folderAlias)
makeAliasList(folderAlias, sortedFileNameList)