According to W3Counter[1], the iPhone OS has 0.83% market share among web browsers. Even assuming every single user is using the built-in browser, that's less market share than even Opera.
W3Counter is just one way of measuring/estimating general browser usage world wide, with the emphasis on "general". It may or may not be relevant to your business needs. The only way to know is measure what is hitting your site(s). In our case we are now getting 17.2% Safari (and growing); and we don't even intentionally target iPhone.
