See the question and my original answer on StackOverflow

If you're talking about SQL Server as the backend, it depends on the server version. Here is a good article: DO.NET and System.Transactions

I quote:

The TransactionScope is smart enough not to create a fully distributed transaction with all of its overhead unless it is necessary. If the resource involved in the transaction is volatile, everything will stay in memory and the Microsoft distributed transaction coordinator (DTC) will not be involved. If the resource is a connection to SQL Server 2005, the DTC will not be involved, and SQL Server 2005 will handle the transaction. This is equivalent to opening an internal SQL transaction. If SQL Server 2000 is used, it will resolve to using a full DTC transaction.