You email client must be compatible with vCalendar format :

Here is where I receive with Outlook

There is some strange behavior on Gmail :

I tested on demo : I retrive an mail with invite.ics file( that can be opened in Outlook)

And on anoster instance : the format is correctly interpreted.

I'll have a look to understand why this double behavior, but what is sure is that format seems correct.

I'll received your 3 attempts, but still empty.

I'm using Outlook 2003 SP 3.

Attachement is message send at 15h19.

In fact the format in not vCalendar but iCalendar (invite.ics file).
And ics files are taken into acocunt only since Outlook 2007.

As Microsoft will not support Outlook 2003 after April, 8th 2013, I hope you will soon migrate...

To complete the anwser, please note that Gmail correctly displays invites only when they are sent to a gmail address.
If you retreive in Gmail your mails from another POP3 account, the invite will not be displayed as a calendar meeting, but just as an ics attached file.