Apple's bug reporting system, known as "RadarWeb", is available [here]. To log a bug, you must have an ADC membership. A free Online membership is sufficient to log bugs, and can be obtained by signing up [here].
Unfortunately, RadarWeb does not allow you access to comments logged on your bug by Apple engineers or a way to search for the bugs logged by other developers. In addition, if your bug is closed as a duplicate of another bug you will not be informed when the status of the other bug changes.
Even so it is always worth logging bugs against APIs, documentation, or applications that you believe could be improved - the overhead of closing a bug as a duplicate is small, but receiving several bugs on the same issue can help Apple prioritize their efforts.
When referencing bugs in email, include a leading "rdar://" prefix:
OK, I have logged this as <rdar://4234682> - I'll work around it for now.
Apple engineers have an internal tool which recognises "rdar://" URLs, allowing them to click these links within your email and go directly to the bug.
An alternative form would be <rdar://problem/4234682>.