I upgraded my GSII to 2.3.6 when it was released and couldn't keep the battery charged at all. It drained fast and hard to the point that the phone was hot to the touch.
I found a stock 2.3.4 rom on the xda forums and flashed back. All was better.
I then forgot about the upgrade while waiting for a patch to cure what ailed it. I never saw anything come about. Then one day, i had an alert show up on my phone that said there was an update.
I plugged it in, started kies and updated, then thought, oh crap, did i do it again? I checked the version number and see enough, 2.3.6.
I was starting to look for the rom I had flashed before, but couldn't find it. So, I then decided to let it ride to see if anything was fixed. Lo and behold, no wifi issues. The battery lasted all day and then some.
After upgrading to ICS recently, i can honestly say that i have seen none of the battery drain issues on it either, so I guess it is just hit and miss on some devices.
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