http://www.mousesavers.com/dltickets.html
Using Walt Disney World Tickets at Disneyland
Interesting, unpublished fact: you can use any unexpired, leftover days on a Walt Disney World ticket at Disneyland or DCA! Present the Disney World ticket at the gate on arrival -- they will take it somewhere, deduct a day from it, and give you a one-day Disneyland and/or DCA ticket. You can repeat this day by day until the Disney World ticket is used up. They will not give you a multi-day ticket all at once, even if you have multiple days left on your Disney World ticket. Also, MouseSavers.com reader Danny P notes that any Disneyland ticket obtained from exchanging a Walt Disney World ticket must be used on the same day it was exchanged.
The type of Disneyland ticket you will receive is based on the Walt Disney World ticket you present: if you have a Disney World ticket with the Hopper option, you will get a Disneyland Hopper that allows you to visit both parks in the same day; on the other hand, if the Disney World ticket had no Hopper option, you will get a one-day/one-park ticket and will have to pick which park to enter that day.
This policy has been in effect for at least 5 years, and the information was last reconfirmed by a MouseSavers.com reader as of August 2007.
Disneyland tickets are NOT accepted at Walt Disney World, and Annual Passes are only good at the park where they were purchased (WDW Annual Passes are NOT good at Disneyland, and vice versa).