Starcraft II Q&A Batch 56 has gone the route of a Fansite Q&A, so the developers responded questions submitted by Starcraft II Source (4players.de). In the original game, whenever a patch is released, the past replays become unplayable. The jewel of these questions answered definitely has to be that replays will be compatible across all new upcoming patches. You can read the whole Q&A here.
Some replays were not compatible with new patches in StarCraft I. Are you working on making replays compatible with new patches for StarCraft II?