Just to be pedantic, the Latin preposition in governs two cases: mostly the accusative as an expression of place whither or motion into, but also the ablative with time and expression of the place where. The latter is sometimes called the locative ablative. Latin had a locative case that it lost before it was a written language. Examples taken from Hale & Buck:
1. ut in Galliam venirent 'to come into Gaul' (acc. whither)
2. in puerita 'in boyhood' (abl time)
3. ter in anno 'twice a year' (abl time)
4. in silvis abditi latebant 'were lying hidden in the woods' (abl place where)
OTOH, ex simply governs the ablative case and is used to expressed 'out of'. The use of prepositions and the cases which they govern and how to translate them in English is quite a detailed and complicated affair and takes up a lot of time once one has learned the rather straightforward declensions and conjugations.
It is interesting to note that Old English, which had 4 cases, has both in and into both of which governed the dative. They expressed place in which and whither respectively. In OE, út was an adverb (as it is sometimes in German), and I have seen út of and út tó.
The question I've always pondered is: are cases slowly made obsolete by the use of more and more prepositions? or, as wsieber suggests, do prepositions come to be used more and cause the cases to wither away? Chicken or egg?
