This has now happened to me TWICE! ACK!
The first time it happened was when I upgraded to Lion a few months back, and was more or less forced into setting up iCloud (I wasn't even sure I wanted it but thought I'd give it a test spin, but then couldn't turn it off again without it removing all my calendar entries from my local iCal). As with everyone else, all the old meeting request acknowledgments mysteriously and for no good reason went out and I looked like a complete fool (and btw, for those that are suspecting Exchange, it isn't just an Outlook/Exchange problem because I'm NOT on Outlook/Exchange, but I interact with customers that are on Outlook/Exchange and who send out meeting requests). I didn't know how to get it squared away and was afraid to touch anything for fear it would happen again until visiting a 'Genius' at the Apple Store who said that I can't synch with iTunes and iCloud, and I also have to be careful when synching between multiple devices (my wife and I share contacts and calendars from the same account on two separate iPhones). So after a frustrating and embarrassing first experience, I thought I was squared away, but then...
This past weekend, it all happened again. All the old meeting acknowledgments went out, again, to the same group of people that got them the first time! So, not once but twice, all my business contacts got meeting accepted messages from me for meetings that were as much as three years old!
Now after reading Mark J P's post, I'm not sure if it was something I did (I was attempting to synch contacts to my wife's iPhone via iTunes this weekend), or if it was just another glitchy weekend for iCloud and had nothing to do with me, but whatever the case, it boils down to two inexcusable bugs, in my opinion:
1). Meeting Accepted messages should not be sent for ancient meetings, ESPECIALLY when said messages have already been sent.
2). Users should be notified that a message is going to go out, and be allowed to decline it.
After these two totally unacceptable blunders, I wanted nothing more to do with iCloud, but when I went to turn it off, it again threatened to delete all calendar entries (though you can choose to keep your contacts when turning off iCloud, there was no way that I could see to prevent it from deleting calendar entries). So I googled around until I found a thread that told me to:
1) export all my calendars now being held hostage by iCloud,
2) create new local empty calendars, and
3) import the exported calendar data into the new local calendars.
Then, iCloud made good on its threat and deleted all my iCloud calendar data when I shut it off, and I am now able to synch my contacts and my new local calendars the way I used to -- with iTunes -- and I am finally free of iCloud's evil clutches!
I am honestly dumbfounded at how terrible this experience has been. I can't believe I am anything other than a very typical user (that shares one iTunes account for multiple devices and accepts meetings from business colleagues that use Exchange), and that Apple would release something as abysmal as iCloud to wreak the havoc that it has.