Last time when I was in Beijing I've used SIM card (pre-paid plan M-Zone) of CMCC operator (it's largest mobile operator in mainland China).
Information about CMCC services prepaid plans you can find at English version of CMCC site:
http://www.bj.chinamobile.com/index/engsite/
There are different GPRS plans: starting from 5 Yuan (~ $0.7) for 10 Mb till 200 Yuan (~ $28) for 2Gb. Initial cost of CMCC SIM-card vary from $50 - $100 (depend on number of included services).

Suggested place to buy CMCC SIM card is official CMCC centers, because they provide fixed price for SIM card package. But need to note that it was not easy to find English speaking person there.

Regarding Chinese mainland maps: they have very detailed google-like maps (but only on Chinese):
http://go2map.com/
http://map.sogou.com/
But I didn't tried yet to add them to Maemo Mapper repository.