I have just updated the xmlrpc_mw.php script to respect the dateCreated attriubute of the metaWeblog.newPost call, for pLog. Now the xmlrpc client can specify the date that the articles are created on, and pLog will recognize it.
I have posted the updated file at http://www.westbrooks.org/xmlrpc_mw.php.txt
Update: I upgraded the version of the the xml_rpc library that had been installed in plog. I believed that this fixed a bug that was preventing the dates from working correctly. You can get the latest version from Incutio's web site. I then merged the changes that had been made in the plog version into the latest version. Here is a link to the diffs.