something changed after April 07

"Under the H-1B portability provisions, an individual who previously possessed an H-1B may begin working for a new U.S. employer immediately upon the filing of an I-129 petition for H-1B status. Based on an interpretation of this law, many immigration attorneys believed that the H-1B portability provisions applied in scenarios where an individual previously held H-1B status but had since obtained an intervening visa status (e.g. H-4, F-1 or TN).

Following E-Verify’s refusal to verify work authorization for such workers, USCIS has confirmed in correspondence with AILA that the H-1B portability provisions only apply to nonimmigrants who are currently in (1) H-1B status or (2) an authorized period of stay based on a timely filed petition for an extension of H-1B status. USCIS / AILA Meeting (April 7, 2011).

The ramifications of this clarification is that current TN visa workers who had previously held H-1B status should no longer utilize the H-1B portability provisions, and instead should wait until any pending H-1B petition is approved by USCIS before they commence employment with the new employer."

Note: howver, one lawyer commented that this "USCIS guidance is subject to extreme controversy and does not carry the force of law.  Furthermore, that opinion was made in the context of the E-verify program, which not every employer is a participant in."