I am about to buy an iPad, and was using a friend's device recently. I noticed all these multi-colour intrusive square things I'm completely unused to seeing because I've been using Adblock Plus for years now.
What are my options?
Here is an update for the current state of affairs. iOS allows for browser extensions to do this sort of thing, you can find them in the AppStore.
When you install something that blocks content in the built-in browser, you can enable, disable or otherwise manage that addon from the settings app built into iOS.
This is of course part of the OS now, so jailbroken or not, this is available officially.
No need for a jailbroken iPad, iCab can block ads http://www.icab.de/mobile.html And it is a great browser.
One option is to use a different web browser, such as iCab, available from the App Store.
If you really want to use mobile Safari, the following method has been reported to work properly for a jailbroken iPhone or iPod Touch, and should work on a jailbroken iPad:
There exists an app in the Cydia store called AdBlock that aims to fulfill this purpose, but many have found it to be nonfunctional.