With one major exception, Clippy, a free download for jailbroken iPhones/iPod touch, makes copying and pasting text across apps easy and convenient. Here's how it works.
If you've gone ahead and jailbroken your iPhone or iPod touch using PwnageTool on a Mac or WinPwn for Windows, open the Cydia installer app, search for "Clippy" (available through the iSpazio repository), install and hit the "Reset Springboard" button to finish up. Once you're installed, you're pretty much good to go.
Clippy works pretty much anywhere you can call up a keyboard on your iPhone—except Mobile Safari. That's a huge limitation in many ways, of course, but having copy/paste in your Mail, Notes, SMS messages, and (especially) Maps is nothing to sneeze at, and one can hope a Safari fix is on the way. In the meantime, the bookmarklet-based, previously toured Pastebud can at least push your web-copied text around the browser and into email (where, hey, you could copy it with Clippy, if you were feeling extra hoop-jumpy).
In the meantime, you get at Clippy's copy/paste functionality by hitting the ".?123" button to switch keyboard types. At the top of that keyboard, you'll now see two buttons, "copy" and "paste," naturally. Hit "copy," use your finger to select the text you want to grab, then (Important!) hit "copy" again to grab it:
If it worked, you'll see the first characters of your copied text in the light-grey bar to the right of the copy buttons. Head to (most) any other app, pull up your keyboard, hit the 123 switcher again, choose your insert point and hit "paste," and there you go:
I find Clippy's most helpful use to be in grabbing addresses and pasting them into Maps, which doesn't offer much address-importing functionality on its own (aside from a Contacts hook-up):
If you give Clippy a try yourself and find some handy uses for it, let us know in the comments. Can't believe a device doesn't have native copy/paste in this day and age? We'll kindly ask you hold off, please, because we've heard that exact witticism, oh, once or twice 'round these parts (which isn't to say it's not a fair criticism). Clippy is a free download for jailbroken iPhones and iPod touch models.