Maybe I haven’t looked hard enough, but I couldn't find a way for Notepad++ to handle a .LOG entry like the Windows Notepad does http://support.microsoft.com/kb/260563/en-us, so I wrote this:

Log.py

from Npp import *
import datetime

BufferIDList = set()

def updateLog (args):
    editor.gotoLine(0)

    if editor.getCurLine().startswith(".LOG"):
        bid = notepad.getCurrentBufferID()

        if bid not in BufferIDList:
            BufferIDList.add(bid)

            editor.gotoLine(editor.getLineCount())
            editor.appendText("%s\n\n" % datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S"))
            editor.gotoLine(editor.getLineCount())
            editor.scrollCaret()

def clearBufferIDList (args):
    try:
        BufferIDList.remove(args["bufferID"])
    except ValueError:
        pass

notepad.callback(clearBufferIDList, [NOTIFICATION.FILECLOSED])

# Tried FILEOPENED before, but had problems with it (wrong buffer getting updated, not updated at all or delayed...).
notepad.callback(updateLog, [NOTIFICATION.BUFFERACTIVATED])

startup.py

from Log import updateLog
updateLog(0)