Take your pick:
- Tap the + button and email a link for the page to an address you've configured to be processed by the OmniFocus mail processing feature on your Mac (see help for "processing mail")
- Install the new bookmarklet in version 1.6 of the OmniFocus for iPhone app and use the bookmarklet to transfer the page information to OmniFocus
The first option works well if you have a number of links you want to send to OmniFocus because it doesn't interrupt your browsing session, unlike the second option which transfers you to OmniFocus to complete the entry. In either case, to copy some selected text, you'll need to find a way to copy that text to the clipboard on the iPhone, either to add it to the email or paste it into the notes respectively. That's easy if you are on a page where you can edit (for example, if I wanted some of the text from the page containing this post, I could hit the Quote button to start a reply, and copy), but I don't know how to do it with a web page that can't be edited. You can scribble some notes to yourself about what it was on the page that interested you, so long as you aren't trying to capture something from an ephemeral page, of course.
To install the bookmarklet, go to the settings page, experimental section, and tap the Install button. Read the resulting directions carefully, as you have to do a bit of editing before the bookmarklet will work.