Xbox.com has listed several new games as Platinum Hits, including Fable II (aka Game of the Year Edition), Halo Wars, Left 4 Dead, The Orange Box, Banjo-Kazooie: Nuts & Bolts, Ninja Gaiden II, Kung-Fu Panda, Sonic's Ultimate Genesis Collection, Sonic The Hedgehog, and Sonic Unleashed.
A Microsoft PR agent couldn't confirm the full list of new Platinum Hits (are we missing any above?), but said that if a game is listed as such on Xbox.com, it's a Platinum Hits title. The changes seem to have taken effect on major retailer sites, as GameStop and Amazon now reflect the discounted Platinum prices (if not always the the Platinum Hits branding), which range from $20–30.
With Orange Box requiring only one Jackson ($20), it's now literally a crime to not own it. Conversely, if you're dead-set on committing a crime, buy someone Sonic The Hedgehog or Sonic Unleashed at $20 each.