A new Knowledge Base article on the Apple Support Site, which was modified and/or added hot on the heels of my needing an iPhone replacement because of this very problem, addresses the unresponsive iPhone touchscreens.
The simple article, which is basically a list of things you can try to see what is wrong with your iPhone, really makes it clear and simple that "moisture" might be the cause of an iPhone’s touchscreen not working properly.
When I visited an Apple Retail Store to get a new iPhone, the Apple genius stated pretty plainly to make that "moisture seems to be the cause of most of these I’ve seen".
I recall, more than a few times, using my iPhone just after playing basketball where my hair and face were covered with sweat. I always try to dry my face as best as possible before answering a phone call, but I can guarantee you that I’m going to be trying much harder or not answering the phone at all when I feel like I’ll be getting the screen wet on the iPhone.
Also something I found interesting about the article was that it never suggested to reinstall the iPhone’s operating system or to do a full restore, as I had done when I was troubleshooting the problem. This leads me to believe that they have nearly never seen this being a software problem and have always found this to be a hardware problem. I wonder if this trend is picking up now that many of the day-one iPhones are a year old now.
Source: Apple: iPhone: Troubleshoot Touchscreen Responsiveness.