2D Boy has submitted an iPhone version of World of Goo to Apple for approval, meaning it'll be available any day now. The regular price will be a dangerously impulse-buy-friendly 2.99 -- but it'll be just 99 cents for the first 24 hours.
And if you already have that iPad version, you already have the iPhone version, in a way. 2D Boy is making the iPad app universal, so it'll be freely playable on iPhone. If you have an iOS device, it's basically a foregone conclusion that you're about to own World of Goo.