This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
In the past, I've had problems with Struts not dealing with overloaded methods properly in ActionForm beans. I'm not sure if the bug has been fixed or not, but I always make it a practice to name the indexed getter and setter something different than the non-indexed ones.
In your example, I'd have a getSubmissions and setSubmissions method to deal with the entire collection, and a getSubmission and setSubmission method to deal with a single indexed entity.