I believe they are going to release it every year. Though I did see from Brian on a different forum that it's the same as the Lambic blend but with different ratios of everything.
It'd be interesting to do a split batch one with each to see if there is a noticeable difference. I still have a pack of the roselare that I'm going to use real soon, but I was going to do a split batch and the other carboy was going to get the White Labs sour blend
Here's the quote from Brian:
shhhhh!! here's a secret....the Wyeast 3278 Lambic Blend is the same stuff as the Roselare, just slight variations on the amount of the brett strains.