Absolutely! This is our ideal plan, and we were planning to do it starting last March, but I chickened out. If the bus to/from daycare is pretty reliable and comes pretty frequently, I wouldn't hesitate.
E has taken the bus with H a couple of times and LOVES it. My hesitation has been with reliability of buses coming from downtown through SoDo during Mariners/Sounders seasons. If that weren't a factor, we'd all still be taking the bus. As it is, we all carpool right now, meaning we spend ~$100/month on gas. My company pays for my parking (they are horrible at incentivizing public transit usage).
Omg I would not do this! Can you imagine a random tantrum on the bus? What if he decided he didn't want to be carried and preferred to run around? I would pick a DC closer to home and take the bus toddler-free.
Says the girl who has never lived anywhere but suburbia.
FWIW, I've never seen a kid have a meltdown on the bus, and there were a few on my commute, usually--everything from young infant to pre-school aged. The only "bad" behavior I can think of is a kid repeatedly pulling the stop cord, and then whining when her dad told her not to do that anymore. The bus driver backed him up, and she didn't argue anymore. I used to read Bus Chick's blog all the time when it was on the P.I.'s site. I started reading before she had kids, and now she has two. She had great anecdotes and tips for busing with babies/small kids. www.buschick.com/
I think with a pre-walker, it would be easier to contain. And anyone past 2 or 2.5...when they understand a little bit more. But for now, tantrums aren't really controllable without distractions, and you don't have a lot of options on the bus. A "time-out" off the bus would be a treat, really....fresh air, new things to see. I don't know if that idea would work well....but again, suburb girl here.