I think this was discussed in another thread elsewhere, but I found two ways around this:
The first involves editing the url once the mobile version has loaded. Many sites have /iphone/ or /mobile/ as part of the url. I just delete this section and hit reload, and most timers this works.
The easiest way though is to get the app 'Atomic Browser' in the app store. It only cost a couple of quid (their is a lite version) but allows you to override websites that direct you to their mobile versions, and has full screen browsing, tabs, and numerous other settings. You can also set it to mimic Desktop Safari, Firebox, Ie6, and various others. I only had it a day, and have made it my main browser.
Saves a lot of woe, and is worth checking out.