Depends, have you had it in for service and did they open the dash possibly causing something to go loose. If so then they should take care of it.
Have you opened your dash up at all for any MODS, that may have caused this?
A rattle in the dash is covered under the base warranty but will not be covered under any extended warranty. They are limited to the amount of time they are permitted to spend on any squeak or rattle, and they likely will take stabs at fixes in the hopes they get it. If it always occurs so that they can hear it when they have it, you are likely to get it fixed. If it is intermittent, then it may take several trips.
One thing to understand is that when they start taking apart the dash etc., you are likely to get some new rattles and/or squeaks, and these too will be covered under warranty.
My friend's 2008 Toyota Tundra has been in and out for dash rattles for a few months now and I know that they are a pain in the butt!!! Good Luck!!!!
No, the dash has not been taken apart for anything. Luckily I haven't had to take it in for any service. It does have to go in for a scratched plastic panel between the driver and passenger door (they scratched), so when they fix that I will have them look for the rattle. It all depends on the road your on to hear the rattle so hopefully they'll be able to hear it.