import datetime

from itertools import groupby
from operator import itemgetter

data = [
    [datetime.datetime(2013, 4, 10, 12, 0), 23.4],
    [datetime.datetime(2013, 4, 10, 12, 0), 25.81],
    [datetime.datetime(2013, 4, 10, 12, 0), 25.08],
    [datetime.datetime(2013, 4, 10, 13, 0), 18.49],
    [datetime.datetime(2013, 4, 10, 14, 0), 20.74],
    [datetime.datetime(2013, 4, 10, 14, 0), 25.86]
]

def ipromedio(i):
    sumar_y_contar = lambda (c1, t1), (c2, t2): (c2, t1+t2)
    n, total = reduce(sumar_y_contar, enumerate(i, 1))
    return float(total) / n

def grupos_promediados(d):
    grupos = groupby(d, itemgetter(0))
    for fecha, elems in grupos:
        yield [fecha, ipromedio(e[1] for e in elems)]

print list(grupos_promediados(data))