I wanted to put post this because for weeks I have been searching for a solution and up until today, I have been unable to find one. Here is my situation;
I use the tethering option on my iphone 3gs to access the internet via my laptop. I use an app called MyWi that allows me to either connect using a USB connection or turn my 3gs into a wireless hotspot. Where I live, DSL and Cable are not available. We have Dish Network but satellite internet is insanely expensive and they have low bandwidth caps. So I rely on my AT&T 3G network connection for internet. Initially I was thinking I could just use the wireless hotspot on my 3gs, short a simple. The Wii would detect the iphone, but when it would test the connection, it couldn't connect. Doing some research, I thought maybe I could share my laptop's connection by turning it into a wireless hotspot. Again, same issue. It would detect the access point but would not connect. Later I learned, after further research, that these hotspots are ad-hoc networks (device-to-device) and the Wii cannot connect to these type of networks. I read about a Nintendo Wifi USB connector/dongle but it looks like those have been discontinued.
Finally after continually researching for about 3 weeks and ready to give up, I FOUND THE SOLUTION! There is a program available for Windows 7 called VirtualRouter and it is completely free.
I do not entirely understand the logistics of how this program works but basically it turns your laptop into a "virtual" router. First, you do need Windows 7, second, you have to have two NIC cards on your laptop. If you can't use your ethernet port on your laptop or if you don't have a USB internet card (or something similar), you would need a second wireless card because one of them will have to be used to broadcast the access point. For me, I connect my iphone to my laptop via USB. It creates a new LAN connection when tethering is enabled.
VirtualRouter is pretty self-explanatory. Choose a name for your access point. I chose "MyWii". Next you'll enter in a password (the Wii will recognize this as a WPA2 key), and then you'll select the LAN connection that connects to the internet. In my case, it was "Local Area Connection 2". Once all that is complete, you can click Start Virtual Router. There is a box below that will show what devices are connected to the virtual router. Now just go into your Wii settings and set up a wireless connection like normal and it should work.
This is the only way I have found that has worked for me. I've tried setting up ICS and bridging connections in Windows XP with no luck. It seems like VirtualRouter is basically doing the same thing but the Wii doesn't seem to recognize it as an ad-hoc network. I suppose that is the difference.
Anyway, here is the link to VirtualRouter if you want to download and try it out for yourself. I just wanted to post this because it sounded like there were several people in the same situation as I was, but no one could seem to give anyone a definite answer. I hope this will help someone and if anyone has anything to add or questions, feel free to reply.