Time displayed in AM//PM format in Activity Stream.

Settings in Administration -> Global Settings -> Look and Feel -> Date/Time Formats:
Time Format HH:mm 08:00
Day Format EEEE HH:mm Wednesday 08:00
Complete Date/Time Format dd.MMM.yy HH:mm 20.Oct.10 08:00
Day/Month/Year Format dd.MM.yyyy 20.10.2010
Date Picker Format (javascript format) d/MMM/yy (%e/%b/%y) 20/Oct/10
Date Time Picker Format (javascript format) dd/MMM/yy h:mm a (%e/%b/%y %I:%M %p) 20/Oct/10 8:00 AM
Use ISO8601 standard in Date Picker ON

According to settings time should be displayed in 24h format only.