if you are running windows XP, just go to your internet connections window, and multiple select the icons for your wireless connection and your ethernet card (if you use dialup, the process is completely different), right click and choose "Bridge connections." Next, open the wireles network list and set up favorite networks. Create a new custom network, and make sure to check the box that says "this is a computer-to-computer [ad hoc] network." Search for it on the Wii, and it should work.
For any mac users curious:
Click the Apple logo in the upper left-hand corner, and open system properties
go to the "internet" tab
In the "share from" pulldown, select your wired connection (USB, firewire,or ethernet)
check "airport" in the list below
search on the Wii, the access point is named "[your name]'s computer" by default