You are indeed correct! Web services by their nature need to work with data constructs that are recognizable/convertible across 'all' languages. [ October 07, 2008: Message edited by: Martijn Verburg ]
Why would you want the client to get an ArrayList?
I doubt the client is going to do anything with the data besides iterate through the array, but if they need an extensible collection, it is quick to make one given an array in any language.