But on a more serious note, I've experienced the problem too. Had my G300 in a jeans pocket as I was moving house, took it out to find the blob (around the size of a 2P coin, centre screen) and assumed I must have put some serious pressure on the screen (since I'd been lifting furniture etc). Was rather upset but when I came to use the phone the next day, it had entirely disappeared without any active intervention on my part. This happened around a month ago, but thankfully it hasn't returned since. It has to be said though, my G300 is actually my satnav and spare phone so it doesn't see much pocket time.
While I'm sure I had a visit from Betty Swollox that day for the manual work, I suspect it is pressure and not moisture related. The ease with which it can be totally removed, without leaving any marks (by many accounts) doesn't seem entirely consistent with moisture build up. Moreover I've owned many phones in the past, some with obvious potential for moisture build up (with dust finding its way under screens, I'm sure this is familiar to some) and yet haven't experienced anything like this previously.
I'm not going to be returning my G300 though, their hard to come by for the money I paid and I've been very impressed by it otherwise