Unfortunately, I believe this SRX210B has experienced hardware failure. While running a show command, I got a memory error. I checked memory, and it had less than 4 MB free. I checked hardware usage and the control plane had very high CPU or RAM (can't remember which). After disconnecting from the SRX, I can no longer connect to it. This may be related to the "control plane would eat all the RAM" described here.
I powered it off for 45 minutes, powered back on, waited 20 minutes to connect, and I still do not have SSH access. I do not have a console cable available right now.
I have purchased an SRX210HE-POE H2 with more RAM to replace this box. The new box will have 2GB RAM and the old box had 512 MB.
I expect to have the new box set up within 6 days. I will come back to this thread if NAT isn't working and continue troubleshooting pings from a host on the 10.0.3.0/24 subnet.