While I would personally love it if DLR got decked out for every significant or notable holiday (I would love to see a gigantic Easter egg or Shamrock where there now stands an enormous Christmas tree), I cannot see them going all out for Valentine's Day. They may add some extra little decorations here and there, but I don't think they will make a whole 'season' out of it as they have with Halloweentime, and it would be too costly and disruptive to business as usual to stick a bunch of decorations up everywhere just for one day. They can't very well do a Valentine overlay for any of the rides...or could they?
However, if any DIS-ers go to DLR and come back with reports of seeing a giant heart where there now stands an enormous Christmas tree, I may have to get there and check it out!!