I've used shuttle services in Rome and Florence where you reserve in advance and they pick you up at the airport. Cheaper than a cab and faster than the metro. Is there anything similar in Barcelona? Staying at AB Viladomat in Eixample. If not, what's the difference in price and timing between a cab and public transit?

The airport bus is cheap and works great. I've used it several times from downtown to the airport. The question is do you have a lot of luggage? If you do, take a cab. The airport bus is great for me because I travel light.

Thanks. My daughter and I travel VERY light - tiny suitcase. How long does the bus take, and how often do they run? Are there express buses to different parts of town (so that a 20-minute journey isn't an hour due to multiple stops)?

I picked the bus up downtown and it made one stop at the Barcelona Sants Train Station. It then went straight to the airport. If you take the bus downtown and then take a taxi, I would think you would save some money. The subways are air-conditioned and quite nice and cheap in Barcelona.

What did you end up doing? When I arrive in Barcelona from San Francisco -- with my toddler and older in-laws, shwew! -- I will want an easy way to get to the Barcelona center and to our apartment we've rented. We're staying very near the central plaza. Also, on the end of our trip, we'll be coming back into Barcelona and need a cheap, easy place to stay to then head to the airport early the next morning. Any advice? Thoughts?

How much is a taxi from the airport?
How much is the bus?

The train is 2 euros/person, the bus is about 3.5 euros/person, and a taxi could be about 25 euros, so I'm leaning toward the cheaper train option, unless we are totally wiped out and want to get to our apartment quickly.

taxi could be more than 30, if you're travelling in rush hour (before 9 am, at lunch and after 7pm). personally being stuck, really stuck in traffic and watching the taxi meter creep up and up is not that preferable to the train.
Two good pages to check out your transport options:
http://www.way2stay.com/transport-Barcelona-en-52.htm
http://www.eurocheapo.com/barcelona/...barcelona.html