a clustered dtc is no longer mandatory for sql server 2008, it was for 2005.

There certainly used to be an issue in sql 2005 where a clustered instance of sql was on a separate node from the clustered DTC and the little MSDTC indicator would show red\stopped.

DTC under Windows 2008 enables you to have multiple dtc objects, typically you would deploy one for each clustered instance that needs it and put it into the sql server cluster group.

DTC is only used for remote calls, linked servers are amongst these.