Why not all the above?

You can hack up a quick library (or find one online) that will take the output of a query and output in the selected format.

Not sure what framework/language you are using but, something like this can be very easily done using PHPs inbuilt functions (ie: get from db and store in array..then output array as XML or JSON etc...).

Open Source as many parts as possible and provide private data to your customers. I think the data format is less important than good documentation of how to use it again.

Thanks for your reply. Haven't thought about providing documentation about that, just adding an option to do it. I don't mind to write tutorials on how to export the client's data to competing services...