We'll be staying in Siena in October and would really like to take a day trip to Monteriggioni. We will not have a car and need to rely on Train or bus. According to the Siena Mobilita site, there is a bus that stops at La Colonna Strada Statale Cassia 5. Looks like a very long walk from the stop to the town (viamichelin plots it a few miles), but I've read in another place that it's a 10-15 minute walk. Train appears to stop in another town & then need to take a taxi.

Does anyone have any first hand knowledge or advice(other than rent a car, LOL)?

It appears to depend on time of day as to where the bus stops. There is an early bus (130) that stops here (travel time 24 minutes):
time stoppointname
08:20 SIENA-VIA TOZZI
08:20 SIENA-V.LE R.FRANCI
08:21 V.N.SAURO
08:22 SIENA-V.LE V.EMANUELE II
08:22 V.LE C.B.DI CAVOUR N.76
08:23 P.DIAVOLI
08:24 TORRE FIORENTINA
08:25 FONTEBECCI
08:26 BRACCIO
08:26 via cassia n.2
08:27 S.DALMAZIO
08:28 LA TOGNAZZA
08:30 FORNACELLE
08:32 IL CEPPO
08:35 SS.CASSIA NORD BIV.POGGIO
08:40 SS.CASSIA NORD LOC.PETRAG
08:41 SS.CASSIA OP.STAB.POLLO C
08:42 MONTERIGGIONI SCUOLE
08:44 M0NTERIGGIONI COMUNE

The same bus (130) later only makes these stops (travel time 18 minutes):
time stoppointname
10:15 SIENA-VIA TOZZI
10:15 SIENA-V.LE R.FRANCI
10:16 V.N.SAURO
10:17 SIENA-V.LE V.EMANUELE II
10:17 V.LE C.B.DI CAVOUR N.76
10:18 P.DIAVOLI
10:19 TORRE FIORENTINA
10:20 FONTEBECCI
10:33 MONTERIGGIONI

Enter your cities and a date (not to far off on the day of week you plan to travel). When you get the list of buses/times, highlight one. The route will appear in a box below. Put your cursor on it to highlight and click. You will get the list of stops.
http://www.busfox.com/timetable/

FYI - If you look on Google maps you can see the school bus stop at the intersection of Via Cassia Nord (SR2)and Strada di Monteriggioni and the Comune stop on SR2 near SR5. That might account for the walking time difference.

Aha, I think I've got it! If I take a bus that lets me off at M Scole I could go up S di Monteriggioni, but if I get off at Comune I would turn left toward S. delle Torri Climate and approach the town from the opposite side.

I had the right bus, but the wrong walking directions from ViaMichelin.com. They had me getting off at the Comune stop (SR2 & SR5), then walking all the way down SR2 to Strada di Monteriggioni, then up into town. Guess they wanted to force me to take a car.

Mille Grazie for the information. I really want to walk the walls that served as Dante's inspiration for the giants guarding the 9th circle of hell. I was despairing I wouldn't get to do it.