I'd like to keep some bitcoins as a long-term investment - store them to a pen drive which I stash in some safe place, or otherwise backup it securely, then remove all traces from my online computer never to worry about this particular wallet getting stolen except by an old-fashioned burglar.
Now my worry is that if I export my wallet with the coins from client program X, which will pack the wallet, the keys, some configuration and other stuff into its own proprietary backup format, then the author ends the project in a year, in five years the OS changes and no longer supports this kind of binaries and their API, and when I finally decide to cash my investment I find myself with a data file no bitcoin client on the market can process.
What is the most portable to survive for years (probably most "raw") bitcoin wallet format? Which software can export/import it currently?