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.)

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.