In case of failover you can get the following error in your SQL Server error log:
Failed to obtain TransactionDispenserInterface: Result Code = 0x8004d01b
This means that the MSDTC service is not working properly or not properly recognized.
If this is the case, SQL Server must be started after MSDTC is started. Microsoft has acknowledged this as a problem what will be fixed in the SP1 from Windows 2003.
If starting in the right order doesn't help, do the actions I mentioned above in an earlier post and start the cluster group, msdtc group and the SQL instances in that order, then everyting should be ok.