There is a solution listed right in the thread. Here it is: Very quick and simple.
In short go to https://www.icloud.com/
sign in using your apple ID information
Click on Calendar
Click on the settings button in the upper right hand corner (looks like a cog)
Click on preferences
Click on advanced
Uncheck "use icloud for incoming inviations"
That will fix the issue. I'm not sure why apple doesn't fix this. It's as simple as them not coding "use icloud for incoming inviations". However it's also a very simple fix we can make.
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rian.Black
Okay, so I've found something:
Until now, there have been many, many Apple product users who've created AppleIDs associated with external email accounts, including Gmail. Now that iCloud has been released, many of them have turned on portions of iCloud or perhaps gone so far as to create iCloud email addresses/aliases under their primary AppleID.
Here's where it gets weird: there's a setting in iCloud calendar preferences turned on by default called "Use iCloud for incoming invitations". What this does is, if an invitation is sent to that primary AppleID (which is most often an external address) or its associated, newly-created iCloud address, it'll get routed to iCloud, preventing (blocking) the invite from being forwarded to that external address.
For example, a user's primary, pre-iCloud AppleID is john1234@gmail.com. iCloud is opened to the public, he enables it on an iDevice or Lion OS X and it asks if him to enter his AppleID credentials (john1234@gmail.com) and asks if he wants to create an iCloud address, which he does, john1234@me.com.
From now on, let's presume he's continuing to use his john1234@gmail.com account, because he's not really sure yet that he wants to make the switch to iCloud for email, etc. At this point, if another iCloud user sends him a calendar invite at his primary email,john1234@gmail.com, Apple has decided it'll route this invite directly to hisjohn1234@me.com iCloud alias, never letting it send out to his Gmail account.
So, assuming he's not yet using iCloud email/alendar, he'll be clueless as to why he never received the invite OR, if he is using iCloud email/calendar alongside his primary Gmail account, he'll probably wonder how in the world an invite sent from a friend to his Gmail account is now showing up in his iCloud inbox/calendar.
Why Apple's enabled this by default is beyond me. At this point, it's preventing me from going to iCloud full-time because I can't be sure invitees will get my calendar events!