Inspired by this post, one of the first things I did on my visit to San Francisco was a #30 bus ride to Chestnut Street. Unfortunately, all I found was this:
I had already passed a T-Mobile store barely 1 block along the street, so I went back there - knowing I'd be settling for 2G - and received excellent service from knowledgeable staff who sold me a microsim, installed it and set up my iPhone to use it. In and out in 5 minutes with working data. "$2 Daily UL Talk Text 2G Web" - unlimited voice, text and data for $2 a day. Calls back to the UK are $2 a minute as far as I can make out, even more than roaming would have been - but at least incoming calls are free which is handy if you are here on business.
There doesn't seem be any daily activation required, I've had it 3 days now and haven't had to do anything. I can only imagine the first time you use it any day, you are charged the $2.
Edge/2G coverage is turning out to be more the sufficient. As well as web/email/twitter, I've also successfully used Skype voice calls and even managed a one-way video. The one-way was probably more down to Skype than the mobile connection.
Similarly to AT&T, it seems that topping up by $100 will keep it active for 1 year. I won't be doing that, so I will lose any remaining credit after 90 days. It remains to be seen whether I can re-activate the SIM later, or whether it will be for the bin.