For me the ForgetMeNot plugin still works in Leopard (10.5.2) and Safari 3.1.1. This is what I've done to make it work.

Open terminal window. In terminal window write command:

sudo /Applications/TextEdit.app/Contents/MacOS/TextEdit

and enter your root password when asked.

Now open Finder and go to /Library/Application Support/SIMBL/Plugins

and right click (or ctrl+click) on ForgetMeNot.bundle and select Show Package Contents from menu.

New Finder window opens. In this new window open Contents folder. Drag and drop Info.plist on TextEdit Icon on your Dock to open Info.plist in Textedit.
Find line:

MaxBundleVersion

below this line you have string (probably 523) Replace string value with 5550.

Save Info.plist and relaunch Safari.

Now Edit -> Undo Close Tab and File -> Unclose Window works perfectly.

Reload windows and tabs when you relaunch Safari option is not working. But you have Option in Safari

History -> Reopen All windows from last session or you can use SafariStand plugin like me.

In SafariStand Settings you have option Restore Method at Launch. Select
Automatically Reopen Last Session (Safari).

Now you have all the options working + you have SafariStand which is great plugin.
I hope this text help someone to get ForgetMeNot work again.

Note: you must open TextEdit from Terminal with root password, otherwise you cannot save Info.plist because permissions.