Ok, I've seen various convoluted methods for uploading mail stored in Microsoft PST files to GMail, none of them are ideal. I have found a tried and trusted method that seems to have worked and is reliable:
1. Open Outlook and add your PST as a Data File:
2. Drag the email from the folder, listed in the folder view, to a folder in your main inbox (Exchange or Local).
3. Close Outlook.
4. Open Outlook Express and import emails from Outlook, or if in Exchange connect to the server using IMAP.
5. Once all your emails appear in Outlook Express, navigate to the email file store (Tools, Options, Maintenance, Store Folder)
6. Copy the relevant DBX files to a safe folder location.
7. Use DBXCONV to convert each DBX file to a MBX file.
8. Use Mark Lyon's GMail Loader to upload the contents of the MBX files to your GMail account.
9. That's it! BTW, I found success using gmail.smtp-in.l.google.com, rather than smtp.gmail.com, which didn't seem to work.