When a note in Stickies is collapsed (double-click on its title bar or type Command-M), you can hover the mouse on the title bar to see a tooltip appear that shows the Sticky's creation and modification dates. For some reason, this doesn't work when the Sticky is expanded.
[robg adds: In my testing, this worked just fine when the Sticky was expanded -- just hover over some portion of the title bar, and you'll see the creation and modification info pop-up. Nifty trick.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070912195554562