In Europe, at least
Ah yes, it appears I glossed over another surprise from Nintendo of Europe's release list the other day (aside from Sonic Unleashed on DS, I mean): World of Goo will be appearing in retail form in the region, not on WiiWare as assumed; the same will go for the PC version there. Gamers elsewhere will still be receiving the title in digital form though.
So, why? Developer Ron Carmel of the game's studio 2D Boy explains it's mostly a money thing, but it's a good situation for publisher and developer:
[The European publisher] RTL [Playtainment] is experienced in bringing retail games to market. They've also dealt fairly and honestly with us and that means a lot to us after experiences we've had with other publishers. In general, Kyle and I favor digital distribution because when you go retail there are a lot of people making money off the game and there's a much smaller piece left for the developer. Retail does have one significant advantage over digital though: publishers are use to paying an advance for a game. After having worked on this game for over a year, burning through savings and getting into debt we were simply nearing the end of our runway and it started being pretty important for us to get some money coming in. RTL was there to help us. We're trying to compensate for the price difference by making some exclusive content for the European version, but we know that some people will still be annoyed that the game will cost more in Europe. We're sad about that, because we just want people to play the game and have fun. We'd be giving it away for free if we didn't need to eat and pay rent 
Regarding price, it looks to be mid-range. Carmel says it won't be a budget title, but he doesn't know if it will be a full price title either; this is up to the publisher.
So, looks as though Pinnacle isn't the only company helping to make bridges between WiiWare and retail. Good on RTL.