It is an iOS 5 issue.  It's because ShareKit is using a method on UIViewController called parentViewController and according to the Apple docs you can no longer use this in iOS 5.  Instead, you must use presentingViewController.
So to fix it in the ShareKit code, go into SHK.m, find the method with signature (void)hideCurrentViewControllerAnimated:(BOOL)animated, and replace it with:
- (void)hideCurrentViewControllerAnimated:(BOOL)animated
{
    if (isDismissingView)
        return;
    if (currentView != nil)
    {
        // Dismiss the modal view
        if ([currentView parentViewController] != nil)
        {
            self.isDismissingView = YES;
            [[currentView parentViewController] dismissModalViewControllerAnimated:animated];
        } else if ([currentView presentingViewController] != nil) {
            self.isDismissingView = YES;
            [[currentView presentingViewController] dismissModalViewControllerAnimated:animated];
    } else
        self.currentView = nil;
    }
}
This works for me on iOS 5.