When calendar items are disappearing from peoples' mailboxes, the first thing I ask nowadays is "Do you own a Blackberry?" The reason for this is simple. If the Blackberry handheld runs out of memory, the first things it goes for are your calendar items.
While it is a good idea to upgrade your your phone's memory anyways, you can also help your memory utilization by properly closing windows such that they do not continually occupy space. It is also a good idea to periodically reboot your phone to clean this up.
I can't tell you how valuable this information is when users come to you wondering what happened to their meetings. Kudos to my coworkers that attended the RIM/BlackBerry/BES training for the initial identification of this problem!