Mine came on when I went over my 5000 mile oil change/service and it
was off after I had it done.  I assumed they reset it for me at the
dealership.  If you're doing oil change, etc., yourself, perhaps you
could talk to them and find out how it's done.  I'd be interested
myself, so thanks for the question.
-
Piper
America.
Home of the free and because of the brave!

Directions are in the owners manual. Don't know offhand what page.

Richard Blood

See last paragraph in http://www.vfaq.net/FAQ-screen.html

- Greg

 Guru Says GOODBYE To Search Engines
http://1stBe.com/goodbye-search-engines

SendFree preferred memberships Free for the
first month! ---> http://1stBe.com/sendfree

At 4500 miles since last reset, the Maint Reqd light will blink at you
on startup.  At 5000 miles since last reset, the Maint Reqd light will
stay on solid.

It's just a reminder by miles that you are due for the next scheduled
maintenance.  It is not smart (doesn't go by time, or do any oil
analysis), just a mile counter. Usually that is an oil change/tire
rotation:  http://smg.toyotapartsandservice.com/

Should your dealer forget to reset it, or you go to an independent
mechanic or DIY, you can reset the light yourself.

Page 122-123 of the US 2004 Prius Owner's Manual (Section 2-5
"Operation of Instruments and Controls: Gauges, Meters and Service
Reminder Indicators", item (j) in the "Service reminder indicators and
warning buzzers" section.)

http://autos.groups.yahoo.com/group/Prius-2G/files/Prius_FAQ.htm#maintenance

page 14 of the User-Guide:  http://john1701a.com/prius/prius-userguide.htm