Well, after another MT upgrade this last week, I broke the install again with the Storable module's breaking with regard to 56-bit vs. 64-bit storage (I think — I plan to research this more later today..). Every time I upgrade, I forget to patch lib/MT/PluginData.pm, so I figured I'd document the required steps here, as it seems I'm not the only one having this problem.
Edit lib/MT/PluginData.pm, after line 9, add the following:
BEGIN {
$Storable::interwork_56_64bit = 1;
}