heh, i had the same idea some time ago and wrote
this userscript to add "(OP)" to the OP's nick on each post.
edit: just noticed that it doesn't work if you use "threaded view" (one post per page with links to other posts beneath (
example) - it'll just add "OP" to the name even though it's not the OP), but who uses that anyway
edit 2: small fix: wrong OP mark when you use a link to a specific post
edit 3: now works with threaded view.
*click*