Not a dumb question, and one that is frequently asked.

Palm Springs is probably too far to even risk it. I believe that according to the rules, if you do not board at your origin as booked, not only does that train's ticket become void, but it cancels everything on the same itinerary downstream. This is not a position you want to find yourself in.

In the past, with manual ticket checks, it was unlikely your absence would be reported and your space opened up for sale before you could board. However, with the recent changes to e-ticketing, handheld iPhones, etc., a conductor can render your whole reservation void before the train makes it to Fullerton. Now, in practice, I think they'll keep it up to one stop, but it's a risk I wouldn't want to take.

Note: If your ticket DOES become void, you'll be credited for what you paid. If that gets refunded, it will be less the cancellation fee. I believe that a full credit is maintained if you keep the credit for a future booking with Amtrak.