I was having all the same problems reported in the earlier threads with the Wifi disconnecting when the phone would sleep, but I think I found a solution that works for me, which was to change the wifi band/frequency selection under the Advanced Wifi menu from "Auto" to 2.4 Ghz only. Now my phone never disconnects during sleep, based on downloading podcasts & Play store updates (which would be interrupted before once I put the phone down)
I currently have a dual-band router at home (airport extreme) with both bands (2.4 & 5 Ghz) running, and my office uses an older single-band G router on 2.4 Ghz. I noticed that the disconnecting would only occur at home, and not at my office, so my guess was that the phone was disconnecting somehow when it switched between frequencies at home.
A software bug is somehow preventing it from re-establishing the connection properly if it switches frequencies until the phone "wakes" again.
Not sure if this is the real solution, or just unique to me & my setup, but for those of you with dual-band routers, give it a try. Just pick one of the frequencies after considering all the locations you use wifi, since you will not be able to connect if you pick 5 Ghz and you go someplace that only has an older 2.4 Ghz router.
I still think this is a bug though, since the phone should be able to automatically switch between frequencies and re-establish connections even during sleep.