Regarding the identity of the company: I am concerned about embarrassing the company publicly. I can email it to you directly. It is possible that this is not a typical agreement at the company. I would be working in a group where IP is of greater than usual concern.
EDIT: Mailed to you at ycombinator.com with my account name in the body.
It seems to me that this Company has this unusual provision probably because it got burned by a former employee who left the Company, joined a competitor, and filed a patent application for an invention that was conceived while he was at the Company. In such situations, the Company should bring an action to contest the ownership of the patent application rather than asking its employees at the outset to execute an agreement to violate the confidentiality agreement of any future employer.
If you explain to them your concerns, they will probably change the agreement for you. My friend had similar concerns about the company he was applying to, but after they talked about it the company cut the offending part from his agreement. I know my friend would've regretted not taking the job, if he knew what he was missing out on. It's apparently a fun place to work.