Anyway, I've been making great progress when I got this delightful error:
Error: 0xC001405C at <
WTF, I asked myself. I haven't changed any code since the last time (the last 10 times) I ran this package successfully. What on earth could have changed?
So I spent a little time digging around online and found lots of references to installing SQL Server 2005 SP1 and post-SP1 hotfixes, but since I'm running SP2 plus hotfixes already this didn't seem particularly relevant. So what had changed?
Well, since it wasn't the code, I thought, it was probably the inputs to that code. The Script Task code in question was reading in name/value pairs from an external file and updating the variable that matches the name with the value from the file. So what had I changed that caused this error?
I had added the same variable more than once to the file.
Apparently if you use VariableDispenser.LockOneForWrite more than once in the same Script Task for the same variable, SSIS doesn't handle it particularly well. I haven't really had time to explore the full boundaries of the problem behavior (and probably won't for the next week, based on what my project schedule looks like) but I wanted to take the time to post this online in case someone else ran into the same thing.
And now, back to my regularly scheduled overtime...
 It's true. I'm working on integrating Windows Teamware Foundation into my data warehouse.
 It's a little more involved than this, but this is the important part. And yes, I know all about SSIS Package Configurations, thank you.