The major reasons they can take a while have been touched on here already. The situation is simply that because:
a ) the DSi is a region coded product
and
b ) we don't know who is going to earn them at Worlds ahead of time
we can't buy them before the event.
To compound the issue, after buying the DSi, we then send it out to be customized. We believe that the custom treatment on the Worlds DSi's should be of the very best quality, but good quality can take time. The upside is that Brad is at the desk next to me currently double checking and packing up the first batch to be shipped and we will continue shipping them out as they arrive here.
I know that these are taking longer than expected but I'm sure that everyone who is getting one will agree that they will be worth the wait.