I believe I found a bug in the replacePages() method of the Document object.
According to the documentation:
If nStart and nEnd are not specified, gets all pages in the source document.
But when I run this method without specifying any of these parameters, just the first page from the source file is being replaced, not all of them.
Can anyone else confirm this? I tested it in Acrobat Pro 8, 9 and 10 and all behaved the same.