If it can play Wii software, it also has the capability to easily play GameCube software. And this new controller has everything necessary for GameCube games (A digital pad, two analog sticks, two triggers, a shoulder button, a start button, and 4 face buttons). So I wouldn't give up hope just because they didn't say anything about it.
The GCN isn't important to most people these days. For a day where the focus is on the critical elements of a console to get people excited, it seems like it would be an easy feature to decide to not even mention. It doesn't matter to most, so why even mention it today when they're out to excite the general public?
At least they didn't come right out and say it won't have GCN compatibility. But even if it's true, at least this thing won't be doing anything fancy like upscaling Wii games, which was confirmed today. So beyond losing out on the space saving benefits of having three consoles in one and perhaps being able to ditch keeping some memory cards around and having to remember what's on each, at least we're not losing any perks by not being able to also play our GCN games on it. If this made Wii games look a lot nicer on a HDTV and had the capability to do the same with GCN software, I'd be much more upset about the lack of GCN support.
Edit - Seems like they might've announced it today. One site that I just found in a search claims that NoA's director of public relations, Marc Franklin, stated that the GameCube will not be compatible with the Wii U.
Let's hope he was just talking about GameCube accessories and they incorrectly made the leap from his statement to assume GCN software won't be playable. I really don't see why something that can play Wii software shouldn't be able to play GCN software (Let us use that tablet controller and give us virtual memory cards) unless it's an additional cost for the disc drive to support the mini DVD's the Cube used.
http://www.nintendoworldreport.com/news/26632