Are you using WPA or WPA2? Also, is your certificate a personal certificate or a root certificate?

The company I work for just upgraded the WLAN to use WPA2 with PEAP and EAP MSCHAPv2.

The certificate I was provided with is a root certificate. When the certificate is installed, it shows up under the Authorities tab but not the User tab (in Certificate manager). I enabled all three Trust options.

For the Connection setup:
page 1: Connection type = WLAN
page 2: Network mode = Infrastructure; Security method = WPA with EAP
page 3: EAP type = PEAP
page 4: Select certificate = None; EAP method = EAP MSCHAPv2
page 5: You can enter your login information if you don't want to log in manually each time
page 6: Click "Advanced" button
Other tab: Enable "WPA2-only mode"
EAP tab: Enable "Use manual user name"; enter your "Manual user name"; Disable "Require client authentication" (I thought I had this one enabled initially, but it will now only work if disabled)

This works for me. Let me know if you have any questions about my setup.