EXPIRED 07/10/15 The Loverpalooza Valentine's Day skin set consists of five unlockable skins given out on February 14, 2013 (Valentine's Day)
You unlock these skins by entering the following Shift Codes:
PC/Mac: 5T53B-6K9KT-Z5B65-BTJJ3-655CZ (Expired)
X360: CTWTB-X3TBC-THXX9-C6JTJ-3JKZW (expired)
PS3: CTWTB-X3TFH-WBWZ5-H3KTJ-3JK9H (active)
These codes are still good as of February 14, 2020 Not true
Source: https://www.gearboxsoftware.com/2014/02/happy-loverpalooza/
The skins will not be added to your inventory but instead will be automatically added to your customization station.
See the List of Shift Codes more codes that you can redeem.
Below are screenshots of all the skins: