Hi,
What is required to happen if two (or more) buttons are pressed concurrently? i.e. Act as if only one was pressed (does it matter which?), Act on both, or as if none was pressed? Can any combination be pressed (some can be avoided by using "rocker" switches in pairs for example)?
Personally I'd try a sequence of resistors of say 1k, 2k0, 3k9, 8k2, 16k and 33k through each switch to ground and perhaps 100k pullup. Some of those values might be made with pairs of resistors but you'll probably use less than in a ladder. With care the A/D converter should be able to individually detect any combination of buttons pressed (4 buttons should be fairly easy, a 5th is getting tight with normal resistor tolerances).
Or if you just want to reject two buttons being pressed, use say 1k0, 1k2, 1k4,1k6 and 1k8, then any value detected as less than 1k0 indicates multiple buttons are pressed.
Cheers, Alan.