I have visited the forum from the iPad.. I don't think there's a need to change the format for mobile tablet users. Smartphone users maybe.. but how large of a market is that? I'd think pretty small..
I have looked at the forum on my iPad and it looks fine just using Safari. Don't know about how a smartphone sees it.
I think what HGP might have meant was how many smartphone users are members (or would-be members) of this forum? Sure, smartphones are the latest development in technology but it sounds like your concern is mostly about getting dinged by Google...I assume that means they lower your ratings or something.
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As to an "app" for your other websites.....I use the genuine eBay app for the iPad, but of course I could use the regular Safari approach to grabbing it as well. The app for that has a really useable, clean look and is worth using. On the otherhand, I have a Craigslist app which I need to take off. I'd rather just use the Safari browswer approach to looking at my listings on cList. So it might be a toss-up for any company's app vs. regular approach.
So, one of my thoughts is that two HUGE companies like eBay and Clist might need it, but do you ? We're talking millions of users countrywide for the first two mentioned.