I think it also can be effected by your target audience... For example, techies probably use higher resolution. General web users might be lower. You might aim at the mobile audience in which case, iPhone is probably one of the highest resolution mobile browsers you will find.
The DBP-Site Challenges site only see's about 2.1% visitors with 800x600 (pretty much everyone else is higher). Wherease ThingyMaJig - which gets over 1,000 unique visitors a day compared to DBP-Site Challenges only getting about 40) see's about 4.26% at 800x600. Funnily enough, TMJ also sees 2.73% visitors running at 240x320 (which I assume is an iPhone). These results are all from Google Analytics.
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