You will withdraw your H1 Extension only after you have EAD in hand which hopefully will be before Dec 2011. From that point on you will be on the Job on EAD so there is no need to have a H1 for job offer. In my opinion I485 will not have any issues for this. However if USCIS denies your application for H1 extension before you get EAD then you will be out of status and in the same situation as you would be if you do not apply for H1 and take leave.
Difference is in the former case (apply for H1 extension), you will have extra time (about 2 or 3 months) to wait for EAD and continue to work. However in the latter case (do not apply for H1 extension) you may not want to sit at home for few months just because your EAD takes time)
my 2 cents..