Generally I recommend referring to the head of the topic instead of a particular post. The topic title above the thread links to the topic / page, and as Jeff points out, the little "paper" icon is for a particular post. I know, I know... the icon should be something like "hyperlink", but that's the standard phpBB implementation, and besides, such precision is entering power user territory.

FWIW, if I want to refer to a particular post, I use Firefox's "Copy Link Location" on the post's image. Reminder for you helpful power users: Use the [url=...]descriptive text[/url] tags, not just plain URLs. The native search will find the descriptive text in the [url=...] tags, but doesn't parse URLs for keywords like Google does.