The HKD is fully convertible whereas the Yuan is not so these two currencies cannot be pegged at the moment. Whilst China is acting more and more like a capitalistic society on a personal enterprise level, it is still a communist country at the national political level. The Chinese government is reluctant to forgo control of capital flows to foreign interests. Many financial 'experts' suggest the Yuan will be fully convertible in the next 2-3 years at which point it seems to make no sense having the HKD pegged whilst the Yuan floats!
I am sure 404 will give a better answer;-)
By the way, I remember seeing a study on FOREX forecasts made by market professionals - they were wrong just over 50% of the time. But I must admit to putting more weight on the Oracle of Omahas' missives!