For a solution that will work in most applications under Windows, not only in Delphi, you can use Authotkey (free, autohotkey.com). One of its many features is the ability to expand strings that you type - typically used for autocorrecting typos.
I start all my shortcut strings with a semicolon, since it practically never leads strings I type in real life, so in your example, to insert a comment-date-username sequence, I would want to type semicolon, slash, slash:
;//
The Authotkey script (which you can put in an .ahk text file and add the file to Autostart) would look like this:
::;//:: ; this means: when I type ";//", do what follows
FormatTime, curDate,, yyyy-MM-dd ; the double comma is intended
SendInput // %curDate% %A_UserName% ; variable expansion
return
This produces the following output:
// 2008-12-05 moodforaday
AHK syntax is a little dense, but there is an extensive help file.
On edit: this script could be expanded to apply the correct comment syntax depending on the IDE you are working in at the moment. You would detect active window title, find a signature substring ("Delphi") and choose the proper comment character(s). This way you could type the same hotstring to insert your comment regardless of the current IDE or language. You can also use SendInput to position the caret the way Delphi templates do.