Interesting, unpublished fact: you can use any unexpired, leftover days on a Walt Disney World ticket at Disneyland or DCA!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 Park Hopper 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.
This policy has been in effect for at least 5 years, and the information was last reconfirmed by a MouseSavers.com reader as of February 2008.
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).
IF ONLY it were true for Annual Passes. How many of us would go to both WDW and DLR if we could use our AP? Would WDW lose too much money or would they make up for that loss with money guests would spend on accommodations, souvenirs, food, etc. Many of us would more likely make both trips if we could use the same pass.