There are two possibilities regarding the iPhone’s continued lack of a system-wide copy-and-paste clipboard. Either Apple’s iPhone UI team doesn’t plan to add it, or, they haven’t gotten to it yet.

I saw a couple of links today pointing, incredulously, to this post from Sascha Segan at AppScout:

I got a few minutes of quality time today to ask Apple product
head Greg Joswiak some of the most burning questions about missing
iPhone applications and features.

Why isn’t there cut and paste? Apple has a priority list of
features, and they got as far as they could down that list with
this model, Joswiak said. In other words, they don’t have anything
against cut and paste. They just judged other things to be more
important.