Game reverts to 1.0.0 when updating to 1.5

When I updated to 1.5, the game appears to have reverted to 1.0.0.

I logged in expecting to see no obvious changes, but the texture pack had changed to default. I checked my texture pack folder, and all my texture packs are now incompatible. I then checked the "about minecraft" from the minecraft menu and got "Minecraft 1.0.0 (1.0.0)". The game menu shows 1.5 in the lower left corner and my texture pack is still active there, but it reverts to default upon entering the game.

I've tried force updating, re-downloading, re-downloading and then force updating, and it always comes back v1.0.0.