Getting these plugins to work again in Safari is easy, but there is one tradeoff to keep in mind.
Re-enabling plugin support for Safari is as simple as forcing Safari to start up in 32-bit mode. When you choose "Get Info" on Safari, you'll notice a checkbox in the window that says "Open in 32-bit mode." Check this box.
That's it. Safari will now open in 32-bit mode, and any Input Manager plugins you were using before upgrading to Snow Leopard, like Saft or PithHelmet, should work just fine. Additionally, running Safari in 32-bit mode allows Multiclutch's custom trackpad gestures to work; these weren't working for me when I ran Safari in 64-bit mode, probably because Multiclutch is a 32-bit prefpane.
There's one caveat to running Safari in 32-bit mode: there's a tradeoff in stability. One of the features you gain by running Safari in 64-bit mode is that Safari will "sandbox" plugins like Flash, so if (when) Flash Player crashes, it doesn't take all of Safari down with it. This is because when Safari runs in 64-bit mode, plugins actually run as their own separate processes rather than being bundled up with Safari. But when Safari is run in 32-bit mode, Flash and other plugins work the old way, which means if (when) Flash crashes, so will Safari.
Safari in 64-bit mode: Flash runs as its own process.
Result: far fewer four-letter words in your workflow
Safari in 32-bit mode: Plugins work, but Flash makes it crash
If you can't live without your plugins (and I can't – Safari without Multiclutch or PithHelmet is like a day without sunshine), then the small sacrifice in stability you'll suffer by running Safari in 32-bit mode is probably going to be worth it to you.
Note: if you're running anything that runs off the SIMBL input manager plugin like Saft or PithHelmet, you might have to take a few extra steps to get things running properly in Safari on Snow Leopard. This procedure is specifically tailored toward getting PithHelmet running, but the steps for Saft should be similar (Procedure courtesy of versiontracker.com user tech.bear).
– Look for "<key>MaxBundleVersion</key>" down towards the bottom of the .plist file; now look at the next line. It's the <string> line.
– Change the number to 9999.9
– Save the file