My first article of 2012 has published over at MSSQLTips.com. This one covers the basic technique for using PowerShell to audit for a server role across multiple SQL Servers. It is assumed you have appropriate rights on those servers. You can read the article here:
Auditing sysadmin on multiple servers using PowerShell
One commenter asked about auditing securityadmin and why I wasn't in this article. The article merely shows the technique to automate how to do such an audit using PowerShell. It doesn't go into detail about everything to audit. Of course, if you're running SQL Server 2005 or higher, you should definitely audit securityadmin, because a member of that role can escalate rights to have complete control of SQL Server.