This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
View the Ant documentation for the java task (a built-in task). It permits nested jvmarg tasks, which are specifically described with examples in the link to command-line arguments. Note that these arguments are ignored unless the VM is forked. Here's an example from one of my projects for starting the JBoss application server: