iPhone HowTo: Block ads = faster browsing, less data
I think everyone on the iPhone has an unlimited UK data plan so there probably isn't much worry about how much data page ads take up in mobile Safari. If you go abroad though you will definitely want to minimise the amount of data you suck down. Regardless of data charges, Safari can be frustratingly slow sometimes as the page won't render or scroll until all those display ads are downloaded.
So to speed up your browsing experience and reduce data usage follow the steps on this handy guide at jamesisbored. You will need a jailbreak iPhone to do this as it requires OpenSSH. The thrust of it is: replace the local hosts file on the iPhone with a modified file. This is the same technique many people use on their home computer to block the display of any ads.
The technical reason why this works is that your computer (or iPhone) checks the local hosts file for an IP before it will use the normal DNS request. The modified hosts file has a list of the common ad serving domains and merely gives it a local IP resulting in "file not found". Voila, no ad gif.