Probably. It's on GameStop's website, after all, and last I checked big corporations aren't as prone to throwing up April Fool's jokes as common folk.
As you can see from the image below, there are 15 artists/songs specifically named, out of a promised 40+. And if most of them look familiar, that's because they are, as they're already available on the home console versions.
Shame. On the bright side, the PSP version will have its own in-game store where you can purchase downloadable content, which is something we figured Harmonix wouldn't bother with.
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You can see the partial tracklist (taken from the back of the game's box art) below.
Rock Band: Unplugged [GameStop, via IGN]