Did it in Io (was already doing Euler stuff), pretty similar:

    problem25 := method(digits,
        term := 0
        n0 := BigNum with(0)
        n1 := BigNum with(1)
        loop(
            n0 = n0 + n1
            n1 = n0 - n1
            if(n1 asString size >= digits, break)

            term = term + 1
        )

        term
    )

    problem25(1000) println