MSExchangeTeam released a spreadsheet with Exchange Server 2010 performance counters. They will release a document afterwards but for now you can use it.
By the way, SCOM 2007 R2 and Exchange 2010 MP is much easier :). Also, Rui J. M. Silva started an article series about this topic, you can check this out at: http://www.msexchange.org/articles_tutorials/exchange-server-2007/management-administration/configuring-native-exchange-2007-mpopsmgr-2007-r2-part1.html. The series is for Exchange server 2007 but you can use a lot of that stuff in your Exchange Server 2010.
Seeing that many will be finding a copy of Exchange 2010 under the tree soon (...right?), we wanted to let you know that we have been working to create the performance counter guidance for Exchange 2010 as we have for Exchange 2007 (see Monitoring Without System Center Operations Manager).
Read more at source: http://msexchangeteam.com/archive/2009/12/14/453579.aspx