Messages, call logs, SMS and calendar entries get deleted if the device memory drops too low (i.e. 0MB). This is why you want to save music and pictures to an SD card and not to the phone's memory. Also, don't install unnecessary themes and/or applications as your memory will drop because of this.
As Foxglove said, 12-15MB or more of device memory is good for the phone to provide its best performance, this does not mean that if you drop to let's say 11MB or 10MB that the phone will crap out on you, but some applications may seem slower than usual.