USCIS - William R Yates Memo on 9/23/2005
Item 5. "... assuming the alien is otherwise qualified for an extension of H1-B status, USCIS will grant an extension beyond the 6th year if the filling date of [a pending or approved labor certification application] or [a pending or APPROVED EB immigration petition(I-140) ] is 365 days or more ..."
this refers to 106(a) AC21, I think it covers 1 and 3 mentioned by "black0702", in other words, my opinion, u can still extend H1B beyond 6th yr if your I-140 is approved and I-485 is pending , by 1 year invrements
104(c) of AC21 deals with the 2 situation mentioned by "black0702"
please let me know if my understanding is wrong