What are the most difficult obstacles to overcome when maintaining a centralized structure for global cash management?

One of the obstacles is having your accounts with different banks. You should look to one of the global banks such as JPM/Chase, Citibank or B of A who have operations globally on a central platform. I just spoke with someone from JPM who indicated that they have a centralized platform.

I would add that as you acquire companies and move across borders this makes maintaining the same level of control of cash you fought so hard to obtain difficult. When I had this task as a treasury professional we made many domestic acquisitions, and I found that centralizing accounts payable up from the acquired company was always more time consuming and cumbersome then I thought.

Also, when you acquire companies, you need to be mindful of the bank relationships that exist at the acquired companies before you "centralize" everything as these banks were most likely key partners in making the company you acquired successful.