In hopes that other people will not be as disillusioned as I was when I found the pencil key was missing from my new S60 device, I am posting this answer:
Use the # (hash) key as you would have used the pencil key.
When in an input field where you have text available for marking, position the cursor where you want to begin marking, press and hold the # key, and then use the arrow keys to move the cursor to the end of the section to be marked.
If you're in a field with text already entered and T9 is on and the underline indicates that T9 is working with the word, use the arrow key to move the cursor so the cursor appears, otherwise pressing the #-key has some other effect.