12/02/2009 Update: MiddleClickClose has been updated for 64-bit Safari. More info here.
I have upgraded my Mac to Snow Leopard, and as soon as I loaded Safari, I could see that MiddleClickClose was no longer working. I had already heard from someone that this was so, and I had expected it, so this was no surprise. It is possible to get it working again by right-clicking (or whatever the native OSX clicks are to get the context menu) on the Safari program in /Applications, selecting Get Info, and then checking the “Open in 32-bit mode” checkbox. Once you do that, SIMBL and MiddleClickClose both load, and the plugin works. But you’re in 32-bit mode.
MiddleClickClose is totally dependent on SIMBL. If SIMBL won’t load, neither will MiddleClickClose. The solution is, most likely, to get a 64-bit build of SIMBL, but I don’t know if that’s a simple matter or not. The SIMBL developer has said that he only has a PPC machine running Tiger, so I don’t really see how he’s going to get it running. If he does, then maybe there is hope. If not, your only option is to run Safari in 32-bit mode.
Or use Firefox, which is what I do.