As a end note: with some more patience I would have found this thread, which has a link to some python code that supposedly performs the decrypting.

https://bitcointalk.org/index.php?topic=93040.0

It was short enough to read and verify that it doesn't seem to do anything too malevolent. Too bad I can't run it, since it reports "ImportError: No module named Crypto.Cipher". And finding out how to install python crypto modules -- well, if I had my life savings in blockchain.info's wallet, I just might have the time and patience how to do it...

But the point, I think, is that wallet backups - any wallet backups in the bitcoin ecosystem, not just those from blockchain.info - should be in a easily readable standard format, decipherable by (relatively) standard tools. If you have the password, you should be able to open them even if the original service provider hasn't been around in ages.