How do I set iPhone so it isn't recognized as mobile device when browsing?

The great thing about having a smartphone, and especially the iPhone, is that it has a fully functioning Web browser. There are so many sites out there that still force the iPhone to display and use the dumbed-down mobile sites, however, and without the option to view the regular, full-function site. Contacting the sites' owners about it yields no results. What I'd really like to know is how to make it so that my iPhone is not recognized as a mobile device. I believe the Android has that option; why can't we have it?

The particular reason I'm asking is that I also use Yahoo business for e-mail, and Yahoo does not allow iPhone users to bypass their mobile mail site. Now, I know what you're going to say: just use the built-in mail feature. In fact I do, but the trouble is that I send and receive lots of mail in the course of my business, and sometimes I need to find a mail I sent a month or several months ago while I'm on the road. The iPhone search function can't help me with that unless I spend an hour downloading all of my sent e-mails from the past months first, and even then it sometimes doesn't find what I'm looking for. The mail search on Yahoo's Web site works perfectly (and quickly!) for that, but for some reason Yahoo doesn't allow me to switch from my personal Yahoo mail to my business Yahoo mail when using their mobile site. I can do so easily on their full-function site, but because I cannot hide that I am a mobile user, I can't access this functionality while I'm away.

I've already given my feedback about this to Yahoo multiple times, and nothing has been done. Please tell me how I can adjust this setting on the iPhone so that I can view the full-function sites as a non-mobile user.