Generating all the combinations of a set of boolean variables in Haskell

What you need is sequence :: Monad m => [m a] -> m [a].

In particular, for the [] monad, sequence takes a list of n lists, and produces all n-length lists drawing one element from each list at a time.

sequence [ [1,2,3], [4,5], [6] ] = 
   [ [1,4,6], [1,5,6], [2,4,6], [2,5,6], [3,4,6], [3,5,6] ]

This helps in your particular case because if you have a list of n strings, you can produce the possibilities for each string easily:

map (\s -> [(s,True), (s,False)] ["a", "b", "c"] = 
   [ [("a", True), ("a", False) ]
   , [("b", True), ("b", False) ]
   , [("c", True), ("c", False) ]
   ]

now you just need to pick one from each list to get your propositions holding a truth value for each variable:

sequence (map (\s -> [(s,True), (s,False)] ["a", "b", "c"]) = 
   [ [("a", True), ("b", True), ("c", True)]
   , [("a", True), ("b", True), ("c", False)]
   , [("a", True), ("b", False), ("c", True)]
   , [("a", True), ("b", False), ("c", False)]
   , [("a", False), ("b", True), ("c", True)]
   , [("a", False), ("b", True), ("c", False)]
   , [("a", False), ("b", False), ("c", True)]
   , [("a", False), ("b", False), ("c", False)]
   ]

sequence (map f xs) comes up often enough that there's a name for it:

mapM f xs = sequence (map f xs)
-- or, point-free style
mapM f = sequence . map f

So your desired function is just

allPropositions vs = mapM (\v -> [(v,True),(v,False)]) vs
-- or, equivalently
allPropositions = mapM (\v -> [(v,True),(v,False)])
-- or, equivalently
allPropositions = mapM $ \v -> [(v,True),(v,False)]
-- or, equivalently, with -XTupleSections
allPropositions = mapM $ \v -> map (v,) [True, False]