If you are a AAA member and get one of their room only discounts, there is no minimum stay. A special offer from disney for a room only discount,usually has a 3 night minimum.
I answered your question in the middle of another DISers post, so I'll post it here also in case you didn't see it there
Usually you can use a room only discount code even for just one night (special package codes always seem to have a minimum, usually 3 nights).
Here's an example I just looked this up on the WDW website under special offers April-May ( which is room only code CMB):
Disney's Beach Club Resort
Check In May 12, 2008 Check Out May 13, 2008 Staying for 1 night
Adults 2 Children 0
Room Type Standard View
Tickets None
Package Room Only Special Rate April to May Room Only Offer
Date Rate per Night
Mon, May 12 $ 279.00*