The Solaceon Ruins, like the Ruins of Alph in Johto, are an area in Sinnoh where you can explore and find the Pokemon Unown. In Platinum, the HM Defog was put into the ruins as well. Unlike in generation II, however, there is no puzzle to solve to get more Unown. The Unown are simply just in different rooms.
Near the Solaceon Ruins, however, there's a young child that will give you letter seals if you shown him various Unown.
The Solaceon Ruins, however, are an optional area at best, as it's not really required to go there to progress with the story.
Did you explore the Solaceon Ruins? What did you think of them?
Though to be honest I only went in a couple times (Sort of a SQUIRREL) mentality and explored both times with not much else going on. It had some okay items to pick up so.... Yeah, not something to buy a T-Shirt saying I've been there before sort of place. At least to me anyways.