Ian,
While this specific problem has no mention in the release note(I don't believe it was reported to the devs, as this problem did not occur on most systems, it only came up very few times), I haven't heard of much problems lately between Outlook and PlanetPress 7.5.1.
Office 2007 should be fully supported, and Outlook being part of it, it should be supported as well. Now, I cannot guarantee that your specific situation will be fixed, as there is no mentions of change regarding Outlook in the new versions, and as I've said, this didn't happen with most customers. My advice would be to try the latest version, and see if it still causes issues. If it doesn't, you may wish to open a technical support issue, so that we can investigate this further.
Regards,
Raphaël Lalonde Lefebvre