First of all - take a look at the joint US-China statement - the only thing we have agreed to is a high-level visit. Not a marriage, maybe just a coffee date.

Second - what is this really about? Probably not NASA. More likely this is just a deal sweetener for something else we want, like:
- devalue the Chinese currency (see NYTimes this week)
- improve WMD nonproliferation efforts
- green technology

At the NASA level, which is at best a secondary concern of most US politicos, here are some possibilities:
- hedge against Russian monopoly of US human access to space during the Gap
- engagement may offer US a better view into Chinese goals and means re: space than isolation would
- possibly a veiled threat to NASA: Ares is not the only game in town: get your act together

I am surprised the following report has not been posted or submitted to /.

Future of Human Spaceflight (16-page PDF)

The MIT Space, Policy & Society Research Group took a step back from the "do this" / "no, do that" debate and examined the very questions being posed by the National Academy of Sciences.

The above link has a 16-page document that examines the reasons for a human spaceflight program. The report is compelling, challenging and thought-provoking.

Give it a read!