Previously, Shrek 2 was not available for sale on Vudu, CinemaNow,, or even iTunes. CinemaNow has it listed on it's Back in Window page, meaning it's HBO run is over. It's only available for rent on vudu, but for sale on CinemaNow!

Edit: It's now available for sale on vudu and UV, but it's not in Disc to Digital database yet.