There is no official confirmation yet from GameStop, but the Nintendo fanboy reports about the GameStop and EB Games Wii pre-order experiences indicate that the Nintendo Wii is a sell-out.

A guy at Ars Technica took it to lightly and missed out for instance. The people over at Joystiq have several reports from different EB Games and GameStop stores across the country. Many customers lined up hours before the stores opened.

The number of available Nintendo Wii consoles varied quite a bit between stores. Some reports indicate that about 30 consoles were available, other sources say stores only had around a dozen Wii consoles available.
Not sure why Gamestop has limited the supply. I would not worry yet too much, other stores and also GameStop will offer more Wii inventory. Still it would be better to already have a Wii secured than to continue worry about it.
See also this discussion thread on digg. More Nintendo Wii News.