The ripemd-160 hash of an ECDSA public key as used by Bitcoin has nothing to do with any version numbers and is therefore universal across Bitcoin, Litecoin, and probably nearly all altcoins. This means if I know your Bitcoin address, I can extract the hash and generate a corresponding address with my version number of choice.
On the receiving end, if I know a private key that has been encoded with Bitcoin's mainnet version number (0), I can re-encode that with Litecoin's version number (48) and spend litecoins that someone else, not knowing my private key, could assume I'd have been able to spend (as long as they know I have the actual private key). A merchant could even advertise this fact...that they are showing a Bitcoin address but will accept other coins sent to the same ECDSA keypair.
As a test, I modified a python script to generate a new keypair and show one line each for Bitcoin (address,privkey) and Litecoin (address,privkey).
https://gist.github.com/weex/6435248 The output of this verifies that the same ECDSA private and public keys were used (check it on the last tab of bitaddress.org and liteaddress.org).
Maybe this is all pretty obvious but it was a surprise to me that I can't find it having been discussed before. Maybe because it takes someone looking at two chains to find it. Anyway, I'm interested to hear what the implications of this might be. A universal wallet? Software that looks for or generates actions across chains? Guerrilla marketing of altchains by donating coins to anyone with a known address in any other chain? Incredibly geeky parlor tricks?
-weex
