@Triumph61 You are correct. That problem also exists. It only occurs with the blank 'accept' and 'tentative' messages, but not with 'propose new time' or 'decline'. Also, 'accept' or 'tentative' messages where the user sending the message has typed something in the message are fine. There is a setting you can activate which will act as a workaround. In Outlook (2007), go to tools/options/email options/tracking options/ and tick 'Delete blank voting and meeting responses after processing'. This should handle the problem, assuming it is a server based setting, otherwise you will also need to make a rule. it had not been a problem for me because (a) I had already activated the setting and (b) I don't think I've yet received a meeting response whilst outlook was closed so I don't know for sure whether its server based or not.
@TJAus - you could be having a combination of all issues mentioned by myself, Triumph61 and Mac_es - I find that its best to start testing with a clean inbox on both the phone and the server.