Adapted from this design posted on Craftster. This is how it came out.
I made mine a little flatter (2 blocks high instead of 3) because it fit the glued on hearts around the edge better. (I glued a little heart on each of the pop up struts.)
They're supposed to look like the conversation hearts, and they cover up the white paper. Okay, but I thought it needed...something...as a background. I wished I had printed a very pale background of conversation hearts on the entire card inside BEFORE I cut it. So..I printed it out separately and glued it on. Much better.
Side view to show the pop up mechanism. 
Finished the outside of the card. It's two sided paper that has the dark purple on one side and light on the other. Here is the cover of the card.
Here is the inside with the light purple edging. I thought the card needed to be trimmed a little smaller, too. DONE! 
More Valentine ideas (click on the card title for more information about any design):
Double Heart Spiral Card
Hot Stuff
Pop Up Coffee Cup
Slotted Heart Pop Up
V-fold Pop Up Heart
Nuts About You!
Felted Heart Card