We recently had a client in Kansas City that was upgrading from SQL 2000 to SQL 2008 which included a number of DTS packages that needed to now run in SQL 2008 SSIS. We worked through the built-in package migration wizard which converted approximately 30% to 50% of package steps and related information but we still needed to rewrite a number of functions no longer supported or implemented differently for SSIS. A few resources we found useful during the effort are below.
The DTS 2000 runtime is still available in certain versions of SQL Server 2005 and 2008 so in some cases you can continue to run DTS packages in SSIS. In one article there was a quote stating that there is no upgrade for DTS to SSIS, and the preferred approach when upgrading SQL Server 2000 DTS to leave the DTS packages alone until you have time or new requirements that mean you just write some new SSIS packages and throw the DTS packages away. Sound advice in a lot of cases.