Wii U will launch with a digital store offering downloads of retail games, but with the Wii U disc holding up to 25GB you could probably learn a sport to an Olympic standard in the time it'll take to download Pikmin 3. Nintendo president Satoru Iwata knows this, so Wii U will let you download games overnight.
Iwata told investors:
First, about your comment that digital download sales of packaged Wii U software may not be so attractive to consumers because of the amount of download time, it is true that downloading software with 10 gigabytes of memory cannot be done in an instant today, even with broadband connections. So, compared with the situation of portable gaming devices, where comparatively compact-sized software can be downloaded, we have to ask our consumers to wait for a longer time before the download process is completed. However, consumers will be able to use the Wii U effectively by finding convenient times to download software such as when they are sleeping at night.
The 3DS offers the ability to download titles in Sleep Mode so this isn't unprecedented for Nintendo — nor other consoles, which have offered standby mode downloads for some time — but it does mean you won't be sat tapping your watch for hours on end while Smash Bros. Wii U downloads.
Thankfully Wii U supports external hard drives so you can download to your heart's content.