SIMBL is an interface to make it easier for 3rd party developers to make plugins for Safari. What SIMBL does when Safari is updated is that it detects that the plugins are for an older version of Safari and refuses to load them so that you can continue using Safari. Before SIMBL, older plugins often caused Safari to crash or have performance problems, so SIMBL's behavior is actually a safety valve to let you continue using Safari.
You need to know what plugins you have installed that use SIMBL (PithHelmet, Saft, AcidSearch, etc.) and then you need to watch VersionTracker or Macupdate to see when an update of that plugin is released and then to install the update. SIMBL will then load that plugin.
SIMBL is not broken, it is protecting Safari from older plugins that could cause problems for Safari. Safari will continue to run with the plugins not loaded, just click through the message box. However, you will not have the use of the plugins until you update them.