The user "Nobody" on the Steam Community forums have discovered a way to unlock this, by installing a previous patch version of CSS. (v72 to be exact).
Details can be found here:
http://steamcommunity.com/sharedfiles/filedetails/?id=821480...EDIT - Apr. 2020:
Seems the original guide has been removed, here are the basic steps to be done. I unfortunately cannot confirm if this still works, if anyone gives it a try i would love to hear your feedback.
1. Download the previous version of CSS - v72. I do not have a link, but should be available around the web. To make sure that you are downloading original, non modified files, i would suggest checking the digital signature of the .exe file. Should be signed by Valve.
2. Install CSS and overwrite the files with v72. Make sure that Steam does not auto update.
3. Set date on your PC to Christmas day 2011.
4. Play against some bots, and they should sometimes drop presents when killed.
When done, i removed all files and had Steam redownload the current version, but i suppose a file recheck might do.