The ebuild for electricsheep-2.4 is still masked and the 2.3-r1 version is too old to be used in conjunction with the electricsheep server so I think there is no point in keeping 2.4 masked since the older version cannot be used anymore...
This is the error message you will get with version 2.3: baal@mydotcom baal $ electricsheep the server is closed. please upgrade your client. see http://electricsheep.org. thank you! server reported error for list: blocked
ok, done