In SCOM 2007 R2, if you have the Exchange 2010 Management pack, you may see alerts with the following description:

"Logon to the virtual directory 'PowerShell (Default Web Site)' failed with the following error: 'Connecting to remote server failed with the following error message : The WinRM client cannot process the request. The WinRM client tried to use Negotiate authentication mechanism, but the destination computer (<servername>:80) returned an 'access denied' error. "

The error is caused by an entry made in the WinRM client configuration. Specifically, the TrustedHosts is set to "*".
You can see this in a command prompt by typing winrm get winrm/config/client. The TrustedHosts will show:

TrustedHosts = *

To resolve the issue, set TrustedHosts to a null:
winrm set winrm/config/client '@{TrustedHosts=""}'
Restart the service "WinRm"

At this point, the Test-PowerShellConnectivity cmdlet should work fine.

You can modify a list of computers at once, using PowerShell:

foreach ($Computer in Get-Content C:\scripts\Servers.txt){
    Invoke-Command -ComputerName $Computer -ScriptBlock {Set-Item -LiteralPath "wsman:localhost\client\TrustedHosts" -Value "" -Force}
    (Get-WmiObject -ComputerName $Computer -Class Win32_Service -Filter "Name = 'WinRm'").StopService() | Out-Null
    }while((Get-WmiObject -ComputerName $Computer -Class Win32_Service -Filter "Name = 'WinRm'").State -ne "Stopped")
    (Get-WmiObject -ComputerName $Computer -Class Win32_Service -Filter "Name = 'WinRm'").StartService() | Out-Null