Amrit Bains from the Indigo team (I guess he is a member, though I don't know exactly) explains the basic features of Indigo's transaction support.
He explains that Indigo fully supports WS-AtomicTransactions and WS-
Coordination. The well known and overall established 2 phase commit protocol is available in Indigo via the WS-AtomicTransactions protocol. WA-AT has a protocol framework to enable 2PC between different parties. More details are in the specification.
The strength in Indigo is to begin a transactions in MSDTC and then the same transactions can be sent via a Web Service to a foreign transaction manager and will be a part of the same transaction. Microsoft is extending the MSDTC TM to accomodate the WS-AT protocol.
For a full reference please see his answer in the Indigo newsgroup.
Read: Indigo and transaction support