As far as I know, there's no 64 bit compile of DTSRun.
I ran into exactly the same issue when we moved our app onto an Itanium. We had to get a second server (32 bit) to run the DTS packages on
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability