I believe this is done on purpose. The reasoning behind it, is you do not want anything that you made to be saved. SharePoint reverts to the previous version and ignores any updates made to the item.
The only thing I can think of is for you to write an Event Receiver that fires on ItemUncheckingOut. This should trap the user who performed that step and when they did it. Your code would look something like this (shooting from the hip)
public override void ItemUncheckingOut(SPItemEventProperties properties)
string userName = properties.UserDisplayName;
string userLoginID = properties.UserLoginName;
//Do something with this information
To the best of my knowledge there isn't anything out of the box that tracks this for you, so using that snippet I gave you might be your only option.
Hope it helps