My memory served me right:
(from Wikianswers):
Hebrew had no vowels. This was unsatisfactory to those who wanted to pronounce the unpronouncable name of God. Their solution? By combining the vowel signs of 'Adho.nay and 'Elo.him' with the four consonants of the Tetragrammaton the pronunciations Yeho.wah' and Yehowih' were formed. The first of these provided the basis for the Latinized form "Jehova(h)." The first recorded use of this form dates from the thirteenth century C.E. Raymundus Martini, a Spanish monk of the Dominican Order, used it in his book Pugeo Fidei of the year 1270.
As such, the form "Jehovah" is of late medieval origin; it is a combination of the consonants of the Divine Name and the vowels attached to it by the Masoretes but belonging to an entirely different word.