David S - I believe that there are reports in this thread about Bangkok and that they indicate that it is possible to get a Chinese tourist visa in Bangkok provided that you have transportation into and out of China as well as accommodations though it would also appear that there are some agencies that can arrange evidence of this for a fee.
To the best of my recollection there is no report of anyone getting a a tourist visa for overland travel as you are required to show evidence of tickets into and out of the country. There are some reports of people having vouchers for trains and that this is sufficient but there are other reports that only plane tickets are acceptable. I can't recall the specifics for Bangkok.
You cannot get a transit visa without tickets into and out of the country. By the time you have the tickets, you might as well make the accommodation bookings and get the tourist visa.
30 days seems to be the standard since 14 April unless you apply in your home country. There are exceptions but it does not appear to be predictable.
beyonist - according the to website of the Chinese embassy in Washington, the rules apply to everyone in the sense that even overseas Chinese need to present tickets into and out of the country. For accommodations, one can substitute information including the address of family and copies of their internal documents. US passport holders seem likely to get 30-60 day multiple entry visas good for one year. I don't recall hearing anything specifically different for overseas Chinese.
Ruth