I can't get my sons Nintendo Dsi to connect. My PCs connect just fine though.
Here's a couple of things I though of off the top of my head.
1. If you are using the Sprint Hotspot for wireless tethering, it defaults to only 2 devices. I'm not sure if Wireless Tether defaults to only X devices.
2. You may have security enabled. Basically, no device can connect unless you explicitly permit it. It is under Menu > Access Control
If it is none of those... I have a couple of possible solutions as well.
1. In wireless tether, try using it with different encryption (Open, WPA-2) or changing the Wireless Channel. All of these are in the Menu > Settings tab.
2. This should work 100% if you have a wireless card on your PC (I'm guessing you do, if you are connecting wireless Tether to it). You will USB tether you phone to your computer and get your internet connection from that. Once that is working, use your wireless card to create an Ad-Hoc network with internet sharing. From there, the DS should be able to connect to your computers AP, all backending through the phone.
Hopefully one of those helps.