iPhone calendar alerts are not muted when vibrate switch is on
The calendar alert is something that you asked for so will not be suppressed by the Silent switch. According to the HIG the switch is supposed to indicate that you do not wish to hear unsolicited sounds such as incoming call ringtones.
A firmware update resolved the issue for me. I'm not sure if something became corrupted, or if a setting had changed inadvertently, but it has been resolved.