There isn't much out there to "celebrate" Valentine's day online. After last year's Love Tatoo I would have expected to find more viral ideas playing with the Valetine's concept.
I just found out about this online game by Indian agency Innoken which has launched "Be My Valentine" (which isn't exactly an advergame). In Italy, icecream brand Algida has created a special site for "San Valentino" promoting its Magnum icecream as a Forbidden Fruit.
Virgin Mobile Australia has launched a viral campaign for Valentine's day, called Dr. Date (via Random Culture).
There is something more in the mobile industry. In Germany Mindmatics has created Valentine Loveline for T-Mobile and Douglas Parfumeries and Enpocket has partnered with Sprint and Match.com to present a Valentine's sweepstake (and promote a new mobile dating service).