It's company policy at Activision to exploit franchises. And you can't very well exploit Guitar Hero: World Tour if it's not on PC, can you? So Activision are bringing the game to PC (and Mac!).
Aspyr (who brought Guitar Hero III to the PC/Mac) will be handling the port, which will feature all the on-disc songs from the console versions, as well as the song creator. What it won't feature, however, is the Guitar Hero marketplace you can find on the console versions, where people can trade user-created content.
As for downloadable content, that's a big "maybe" at this point, Aspyr unable to confirm or deny that PC gamers will be able to buy content post-purchase.
The PC/Mac version will ship with a single guitar controller. If you want more controllers, it's compatible with the PS3 instruments, which you can buy separately easily enough.
It'll be out in June.