I have only 5000 miles on my XA and recently I noticed that when I start the car, the yellow maintenance light flashes for about 30 seconds. But it doesn't stay on, so I figure there's nothing wrong.

Is this normal? Has it always been like that and I never noticed? Or should I bring it into the dealer?

yes...it's normal. It reminds you to change your oil at 5000. If ignored it will eventually stay on constantly when you hit 7500 miles.

Here ya go. You dont have to go to the dealership for an oil change to take care of the maintenance light reminder.

http://www.scionlife.com/tech/electrical_reset_maintenance_light.php

other then the tech article here are other threads

http://www.scionlife.com/forums/viewtopic.php?t=7220&highlight=reset+light

http://www.scionlife.com/forums/viewtopic.php?t=3429&highlight=reset+light

http://www.scionlife.com/forums/viewtopic.php?t=2868&highlight=reset+light

http://www.scionlife.com/forums/viewtopic.php?t=2504&highlight=reset+light

there is also a how to in the owners manual