I just entered an appoint for this coming thursday at 8 AM on my Mac Mini with OS 10.7.5. Within 5 minutes without closing iCal the appointment disappeared. I have noticed this before.
Then I noticed that an appointment for Jan 9th had disappeared. I opened my iPhone 4 with the most recent OS upgarade and as i watched Cal synch on the iphon the appointment disappeared right before my eyes!
Gonna try the icloud troubleshooting but this is not good!