I read articles via Instapaper all day every day and I'm not experiencing any problems whatsoever with Wi-Fi. Never have.
It seems to be only at certain times, usually at night. At first, I only noticed that whilst reading a long-form article - after bringing up the status bar from full-screen - my phone had reverted to 3G.
I switched to Settings, Wi-Fi was on but not connected, and I had to physically select my Wi-Fi network to get it to re-connect.
However, after returning to Instapaper and continuing reading (in full-screen, landscape) after a few minutes it has gone back to 3G.
It's not just Instapaper though (Instapaper accounts for roughly 80% of my overall iPhone usage), I've started to notice it happening on other apps, which is particularly annoying when I'm streaming a tv show on iView or Youtube.
Strangely, it doesn't appear to happen if I disable Mobile Data whilst connected to Wi-Fi... And yes, I've done the whole "reset network settings, restore, etc". I'm a bit baffled!
Any thoughts?