Eiríkr Útlendi wrote:
As another 2p in the pot, German appears to be at an earlier stage of this development. Inflected prepositions are pretty clearly contractions of conjunction + article, where the inflection comes from the article. Examples include zur in zur Tur ("to the door", dative feminine) = zu + dative feminine definite article der, or am in am Tisch ('on the table", dative masculine) = an + dative masculine definite article dem.
The phenomenon of preposition + article contraction in languages like German, French, Spanish and Italian seems fairly different from the phenomenon of "inflected prepositions" in Celtic and Semitic languages. "Inflected prepostions" in these languages are either used in place of preposition + personal pronoun sequences, or used before a personal pronoun.
Prepositions are not inflected into their third-person gender/number-inflected forms before non-pronominal noun phrases (at least, not typically or obligatory: I don't know if there are any special situations where this is possible or required), although prepositions may in addition to their inflected forms have special forms or contractions when they occur before an article.
Wikipedia points out that the use of inflected prepositions has some parallels in Spanish forms like "conmigo" and "contigo".