Exchange 2003 SP1 introduces the ability to move public folders, mailboxes, distribution lists, and contacts across site boundaries -- even while still in mixed mode. This is a huge change in behaviour, and prior to undertaking these sort of moves, it's imperative that folks understand a bit about how the process works and about what planning and preparation must be done to make the moves work properly.
Released at the same time as Exchange 2003 SP1 were a number of aids which will help you do the planning and preparation you need prior to (and during) the process of moving these items cross-site:
To do cross-site moves of public folders, mailboxes, contacts, or distribution lists it is highly recommended that you follow the step-by-step information in the updated Exchange 2003 Deployment Tools (updated at the same time as Exchange 2003 SP1 and available for download from the 2003 Tools page at: http://www.microsoft.com/exchange/downloads/2003.asp).
It is also recommended that you have a look at Chapter 10 of the updated Exchange 2003 Deployment Guide
(http://www.microsoft.com/technet/prodtechnol/exchange/2003/library/depguide.mspx) and at Chapter 4 of the updated Exchange 2003 Planning Guide (http://www.microsoft.com/technet/prodtechnol/exchange/2003/library/messsyst.mspx). These two chapters will help you prepare to run through the step-by-step in the Exchange 2003 Deployment Tools so that the cross-site moves will be as painless as possible.
If you don't reference these materials and the Deployment Tools steps, you're likely to have difficulty as the process is complex and has many steps.
One specific blocker that some folks are running into: cross-site moves require a hotfix for 5.5 DS/IS be installed on every Exchange 5.5 server with a public folder store in the Exchange Organization. The hotfix required is the KB.836489 hotfix. This hotfix is actually rolled up into the “May 2004” 5.5 Store roll-up (http://support.microsoft.com/?id=841765), so downloading and applying that roll-up hotfix will be sufficient to meet the prerequisite for cross-site moves. I mention this earlier in my blog at: