It's hardly undocumented. If you look through the articles on 64-bit SQL (on the MS site) it's quite clear that most of the tools don't have a 64-bit version. The DTS runtime is one of them.

I've got a 64-bit server here, and to be quite honest, it's not that large an issue. My main production server is 64 bit (itanium) and I've got a second server (32 bit) which has all the DTS stored on it.

A much bigger issue which I had is that there are very few (stable) OLE DB/ODBC drivers available for 64 bit.