I was having some DTC issues with a client the other day and I stumbled upon a post about MSDTC and the Network Service account on Florin Lazar's blog. Although it didn't resolve my issue it's probably useful information for people migrating from Windows 2000 to 2003 who are suddenly having DTC issues.
Oh, and Windows 2003 SP1 locks down DTC a bit more tightly so that's something to keep in mind if you're “suddenly” having issues.