The Penderwicks might have been the first one that caught your eye, and its appeal is old-fashioned.
And certainly you've been seeing these two everywhere - both stories (one fiction, one not) set in the past:
You might not have seen the one pictured below yet (only partially silhouette, down in the bottom left corner, but certainly of the Silhouette School) since it's not due out from Henry Holt until next April...it's based on one of Grimm's fairy tales called Bearskinner:
The template (old-fashioned storytelling or set in the past) holds for these as well, from the artist David Frankland, who appears to have a thing for pointy rooflines in the distance:
Only a Witch Can Fly (seen at the beginning of this post) is the only picture book I can think of right now with silhouettes on the cover, and it is being praised as old-fashioned. Are there other recent picture book covers that use silhouettes? I'm wondering if these silhouettess are drifting up to us from a love of this well-loved image of Ferdinand the Bull under his cork tree:
The only Silhouette-School cover I could find which represents a completely modern story is this one, designed by Christopher Stengle, illustrated by Dan McCarthy:
And maybe the fact that Marcelo does not seem to be of the "real" world explains the ironic cover - signaling the story of someone who lives neither in the past nor the present but someplace deeply in shadow...?
If you have other samples of recent silhouette covers (I'm sure I've missed some) leave suggestions in the comments and I'll try to update.
[Update #1: Sample of the new covers for Zilpha Keatley Snyder's books, as suggested by jessmonster in the comments, and the new book by Patricia Wrede, The Thirteenth Child ]
[Update #2 - And just learned of another - Margo Lanagan's Tender Morsels, issued as a YA book here, has a crossover version in the UK for adults - that cover is below.]
[Update #3 - Another! Every Soul a Star by Wendy Mass. The story sounds fascinating, too. Here's an interview of the author over at the bildungsroman blog. ]