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?
At the risk of continuing pedantry, nuncle, I'd suggest neither of these (similar?) options exactly describes the likely process...
When studying early English I was taught that the most likely cause of 'case fall' was the abrasion of two variants of language with otherwise many similar stem forms - eg, at the boundary areas contested by the NG tribes who settled either side of the Danelaw. This seems a convincing case to me ;) After all, if you encountered a stranger who used a word the main part of which you recognised as common with your language but with a weird suffix, the chances are you'd latch onto the former and be quite inclined to not register the latter, whilst she would do the same...
The common language would grow to be a simplified stem-base construction. Surely we see similar processes of language loan words getting adapted and simplified, and whole creoles sometimes emerging by similar concentration on the roots of the vocabulary?
If that process of language collision is the main driver of change, it would suggest that perhaps the prepositions get added after the fact, as an aid to greater clarity or sophistication. What thinkst thou?
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