IMO it depends on what your goals are. I went to WDW for 2 days in 2008, I think it was, and I feel like I saw all of Magic Kingdom, about 80% of Epcot, and 70% of Hollywood Studios in those 2 days. People on the WDW board, however, will say there is NO way you can do WDW in less than 5 days/a week/etc. And that is true, but I got a good feel for 3 parks and had a wonderful time.
There is no way you can see everything in the parks in 2 days, and if you only go for 2 days, you will probably also miss a whole lot of the atmosphere. But if you want to see some of the parks and have a great time, you can really do a lot in 2 days.
If you're OK with running from before park opening to closing time both days, and if you're OK with just getting a good dose of Disneyland and DCA (and don't mind missing some things), then you I think you can do 2 days. If you want a more leisurely experience and/or want to see mostly everything, I'd do 3 days, and probably a park hopper if you can swing it. If you want to have your best chance of seeing and experiencing it all, maybe 4 days.
Have a great trip!