It's a lambic, rather than a sour mash thing, but you could give it a go that way, I'm sure sour mash and sour cherry could make for a great beer.
I don't think it has any real cherries in it, it has that sharp medicinal tang from some kind of cherry extract/syrup.
I like it (and Bellevue) as a guilty pleasure from Dan's etc, it's like alcopops for beer geeks :O If you can get your hands on a Cantillion or something that uses real fruit, it's a stark difference, but it also uses literal shedloads of cherries (and not easy to source type of cherry) and sits on a 2-3 year old gueze (blended lambic beers.) so not overly practical for homebrewing.
The best way to get into Kriek really would be to try and get a house lambic happening that you can add to/blend etc. and then age some of it on cherries (or add the syrup like Timmermans)