ETHAN  Since there exist multiple (I counted 4) options, including doing the XCode 64-bit way, here's a summary:

Safari on 64-bit works with this mod from:

http://sweetpproductions.com/safariadblocker/

(Also listed here on MacUpdate)

The modifications of github are from this persons work:

http://burgersoftware.com/en/safariadblock

However, he uses the InputManager method. This has been depreciated and is NOT supported on 64-bit Applications. Apple wants the IM method to go away (google for more comprehensive commentary by some of the folks who have worked more with it).

There are also four or five forks, which I was unable to get working, looking like it had to do with the problem with IM on SL due to 64-bit.

The first mentioned build works because it uses a 64-bit build, using normal SIMBL plugins you can install via ~/Library/SIMBL/Plugins

It also takes care of installing SIMBL, using the 0.9.7a (at this time of writing) SIMBL plug.

I also assume GlimmerBlocker works, since it is NOT an InputMananger but instead proxies all your traffic as if you were on a corporate LAN and having all traffic filtered via a regular HTTP proxy. It's a great alternative solution, using components from the Mozilla Project, and is available opensource as well.

Hopefully this clears up some confusion.   
(Version 2.2r1)

GURUBEZ  Grrat Software, since it works on Safari4 64Bit its the best AdBlocker I ve ever had!!! It block also Flash Ads with a simple right click ...

I had Pithhelmet before (on 32Bit) but I think it will be really hard for him to beat SafariBlock ;)  
(Version 2.2r1)

APPLE_CAKES  Right Click Flash is no longer working in Safari 4.0.3 after the update to Shockwave Flash 10.0 r32.

Lots of annoying flash ads are getting through.  
(Version 2.2r1)

AARGL  Suddenly, yesterday, I've got an error message meaning "impossible to update the list" (really "too many http redirections").

After various tries, I found out that the address of the list has changed:

Now it's https://easylist.adblockplus.org/easylist.txt

instead of http://easylist.adblockplus.org/easylist.txt

But as the "http://" is added automatically, I think there's no way of changing it to "https://"... :-(

Strangely enough, while the auto update gives an error (and so do the "update now" button), if you click the "change..." button and validate without modifying the address, it works! (at less it seems)  
(Version 2.2r1)

TOMP  I got it to work in 64-bit Safari:

- Download the updated SIMBL, uninstall the old one, install the new one. Reboot.

http://www.culater.net/software/SIMBL/SIMBL.php

- Download the 64-bit Safariblock bundle, put in ~/Library/Application Support/SIMBL/Plugins/

http://www.filedropper.com/safariblockleopardbundle

- Make sure Safari isn't in 32-bit mode (command-I on Safari).

I followed the instructions at http://code.google.com/p/safariblock/issues/detail?id=115 particularly comments #26 and #33.

(Also, the new SIMBL caused a minor problem with SafariStand, which is fixed by a new SafariStand version.)  
(Version 2.2r1)

ISLANDMACMAN  This appears to not work with Safari 4.02 - Flash 10 ads and others are getting through.  
(Version 2.2r1)

THEKIDFROMTHEREDBANK  Not working with Snow Leopard. Any info from the developer on this?  
(Version 2.2r1)

JEFF7  its working for me again in safari 4  
(Version 2.2r1)

JEFF7  Doesn't seem to be working at all in version 3...it used to, but after going back to 3 two days ago and installing fresh no ads are getting blocked.

WTF?  
(Version 2.2r1)

TOMP  In Safari 4.0.1, right-click flash blocking seems to have stopped working, which is unfortunate. I'd love to see that restored. Otherwise it's a very good adblocker.  
(Version 2.2r1)