I used to work for an organisation that campaigns for free press, etc. so I know a bit about how it works.
ninedragons says from her experience The Great Firewall is overplayed in western media, but I strongly disagree. Rather, foreigners often do not experience the full effect of it. That is because we often benefit from exceptions to the rules.
For example, hotels full of foreigners have VPNs (Virtual Private Networks) that act as funnels outside of the wall. Places that are known for being mostly foreign will also have these 'free passes'. Often foreigners are using company VPNs from back home - so again, circumvent the wall and are completely unaffected by the censorship. They'll come home and say "No problems at all," which is their experience, but not necessarily the reality for the vast majority in China. Furthermore, if you navigate the web in English you have a different experience than if you do so in Simplified Chinese. Words banned in Chinese are not necessarily 'caught' in English. Content within paid subscriptions is also often excempt. So, if you are a subscriber to The South China Morning Post, you can access articles online that have been redacted from the print version circulated on the mainland. The idea is that the censorship is to control protect the 'masses' not 'the elite', thus being foreign, affluent (by local terms) and English-speaking gets you past most of it. The short answer is that you probably won't have to worry so much about it (but may need access to a VPN).
Even for the 'masses' it is fairly straightforward to find ways to 'break through'; so the firewall is much more of a cat and mouse game than anything else. For many, finding ways around it is about having a less intrusive online experience, but for some people - human rights or democracy activists, or anyone purposely or inadvertently shaming the government, it means high risks. Blogs can be shut down overnight, people can be interrogated for posting embarrassing photos or comments - and much worse: Huang Qi, for example
Imagining that you are not over there as an revolutionary you have little to worry about. However, the one comment above that I would most definitely heed to is about spying. If you recall the massive cyber attacks end of last year - most of the targets were commercial: Google, Adobe, etc. I would say you are more at risk of commercial Internet Security breeches in China than elsewhere as there is already a sophisticated infrastructure in place to do so, and it often happens with the tacit approval of the government, which has an appalling record of respecting international Intellectual Property rights, and which, in a sense, benefits from this behaviour. If you have anything you think could be worth stealing - be careful with it!