I am classifying this as a bug as it only affects the latest snapshots which added the post-processing effects:

I can no longer force the game to use 4x anti-aliasing with super-sampling. This is important to me as textures like leaves will "shimmer" as the viewpoint moves.

When I revert the game version to 1.6.2 or 1.6.4, anti-aliasing and super-sampling function as intended.

When I return to the latest snapshot versions, AA and SS again no longer function.

I have attempted forcing AA and SS through the nVidia graphics drivers through several methods: globally, for minecraft.exe, minecraft launcher, javaw.exe and java.exe to no effect. "Disabling" or "Enabling" post-processing effects in 13w38a also has no effect (this button was removed in 13w38c). I have rebooted my system and game several times in several configurations.

The new "super-secret" filters are inadequate for anti-aliasing support. The provided FXAA filter only blurs the image, and the 2xAA filter adds inadequate anti-aliasing (shimmering) and adds minor blurring of the image. The 4x mip-mapping helps somewhat, but isn't enough to fix shimmering for nearby objects.

Edit: 10/10/13 For 13w41a - There is still no improvement to AA, and in fact things have gotten worse as there are now lines between every block starting at moderate distance.