I was able to find my missing transactions in MtGox's feed, and from a cursory investigation, it seems like the sky might not yet be falling. (I.e., the cause might not be MtGox's insolvency, but rather incompetence, which means we might eventually see our Bitcoins.)

It turns out that one of the outputs that MtGox tried to use to produce my transaction was a recently generated coinbase output which is not yet 100 blocks old - not even at this point, let alone hours earlier, when MtGox tried to spend it. Apparently, people are mining to MtGox addresses associated with their accounts, and MtGox tries to spend them after a handful of confirmations rather than 100, which coinbase outputs require.

There might be problems with other previous outputs, but this would definitely cause a transaction to stall.

I'm somewhat confident right now that this is a big technical snafu based on old bugs that MtGox hasn't bothered fixing in a while (such as the known coinbase bug and the known duplicate spend bug), but which snowballed now that many people have panicked (like I have) and tried to withdraw.

Hopefully, I'm right about this, and we'll eventually see our Bitcoins.

It's still gross incompetence and unimaginable customer service, of course.