this would actually be a really cool app for the iPhone, but I think it would need tweaking a bit. Part of the greatness of StumbleUpon is that the pages load quickly on your computer and you can see whether or not you like it, then quickly stumble to a new page, and keep going. The iPhone's browser will be much slower (even on WiFi it's dramatically slower than my computer's browser, and on Edge or 3G it will be slower still) It would suck to waste a lot of time stumbling to a new site, only to find out that it sucks and you just used up battery life and time. Instead, maybe they could make a dynamic page that could give a list of sites in your given category and you could choose what sites you wanted to try out from that. Maybe even having a screenshot of the front page of each site load to show what it would look like.
Just an idea at least