FYI, from what I have seen thus far Active/Active on the SRX's (JSRP) is significantly different from Active/Active on the Netscreens (NSRP).
No matter how you configure HA with JSRP only 1 routing instance will be active at any time and it controls the forwarding on both cluster members. The '/Active' part of JSRP seems to refer only to the fact that data packets can traverse the passive RG0 cluster member whose forwarding-table is built by the active RG0 cluster member.
From the NSRP world I had always thought of Active/Active as two (or more) highly available routing instances running on the cluster, one on each cluster member in the normal state. From this perspective 'Active/Active' in JSRP is more like an extended chassis than Active/Active from the NSRP world.