This is a long story. For the impatient, the short answer is that there will not be a PithHelmet on 64-bit Safari for some time.
One of the primary directives for SIMBL and PithHelmet was to "first do no harm." The idea being that hacking up someone else's application should only improve it. Under Snow Leopard, many things have changed and they are almost universally considered improvements.
Unfortunately, one of the casualties of these improvements is the relative ease with which applications (in this case Safari) could be hacked after the fact. PithHelmet has grown into an incredibly complex plugin. It hooks into so many subsystems inside Safari, each of which change from time to time. Under Snow Leopard, so much has changed that simply modifying the old code is no longer viable. It will be better to just start fresh.
While I am convinced this is the right approch, this is complicated by the fact that there are many changes to GCC, the dynamic linker, the Objective-C runtime, SIMBL and Cocoa itself. This is a tough task and it will take some time, but I feel that releasing something that doesn't work well would be a bigger mistake.