This is a gray area for exempt to cap-subject H1b transfer

Facts: Conservatively, many postdocs wait till OCT to work. Some agressively start working upon filing.

Here are some interpretations from Murthy lawer.
It seems AC 21 can be used to work in company after H1 filing, but have to stop working once H1b is approved with starting date on OCT. However, the applicants may not be considered a violation of the rule if he continues working upon H1B approval. That's why many lawyer say YES for university H1b holders.

Chat User : Can a person with an exempt H1B start working if his non-exempt visa under the masters quota was approved for the fiscal year 2007 - before October 1, 2007?

Attorney Murthy : Under a strict reading of AC21, a person is allowed to start working for the non-exempt H1B employer simply upon filing of the new H1B for the cap-subject employer, even if the earlier employer was a cap-exempt H1B employer. However, upon the USCIS approving the H1B petition with a start date of October 1st, one is required to stop working until the start date of the new H1B petition, if the approval contained the I-94 card attached at the bottom of the approval notice. Although this matter is not clear under the law itself, the USCIS, in various meetings, has confirmed this interpretation under the wording of AC21, and until the USCIS issues regulations, it seems like a viable option to consider.
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Chat User : I am on a university H1B (quota exempt). Is it better / easier to apply for a new H1B visa (U.S. graduate quota) or to transfer my H1B if I want to take a new job in the industry?

Attorney Murthy : At the present time, per the USCIS count, there are still H1B numbers remaining for the U.S. masters or higher degree quota, so if one is switching from a cap-exempt H1B employer to a cap-subject H1B employer, the employer should file the new H1B petition immediately, since the USCIS predicts that the H1B cap will expire before the end of this month (January 2006). Either way, upon filing the new petition from a cap-exempt H1B employer, AC21 law seems to allow one to start working for the H1B cap-subject employer, but that person must stop working upon the approval of the H1B petition with a future start date like October 1, 2006. In other words, while the H1B petition is pending, H1B portability would appear to protect an individual, allowing that H1B candidate to work legally under a strict reading of the AC21 law. The USCIS has agreed to this interpretation in various national and regional AILA conferences.