Found some useful, but they are for invision, but maybe you can 'convert' them to vbulletin, or their idea.
Ok, so lets start:
1 - found a mod that would create a button with a "thank you message" - aka thank-o-matic... but then again, maybe it'll attract only more lurkers.
http://mods.ibplanet.com/db/index.php?mod=1092
or
http://mods.ibplanet.com/db/index.php?mod=1089
2- The one I've spoken of: (redirects)
http://mods.ibplanet.com/db/index.php?mod=1023
See his explanation for using this, though I doubt those mathematics.
3- About that google thing... no, I got it the other way around. Acording to this guy, Google doesn't like query strings, and he proposes to make the url more google-friendly...
http://mods.ibplanet.com/db/index.php?mod=775
4- Banning by cookies...

It'll only work on lames, though.. but that's what we want right?
http://mods.ibplanet.com/db/?mod=504
5- Awards to best cracker, or something... Maybe they should be given only by mods, supermods, or admins, and invites to gold would be on 9999999 awards given,

http://mods.ibplanet.com/db/?mod=475
I hope this helps somehow to improve the board. I'm sorry the examples are for invision, and I would make them myself for vbulletin, but not such an expert in php.
Number 2 seems useful if you're having bandwith problems.
And Jamis, would that really be an 'improvement'? lol It would just make you hate lurkers more, for not replying...
Duncan