This affects also Creative Mode, Single Player and Multiplayer (but it seems to happen only on my computer), and started a few days ago when I forced the update of Minecraft (I already had the version 1.4.7).
If I try to destroy some blocks of natural snow or the dirt below it, the graphic freezes; background music still plays (not the sound effects!), but Minecraft doesn't respond (I have to ctrl+alt+del and close Java). This happens every time, sometimes when I break the first block, some other times it happens later...
I think the game still runs, but the charatcer can't be moved:
once I noticed that, after having restarted the game, there was a hole close to me caused by a Creeper explosion that must be happened after the crash.
Java still uses the same amount of RAM: around 600.000 KB.
Also, it seems the crash happens just when I click on the snow, or when the block below is destroyed, and I don't recive any message of error.

Edit:
I just tried to set the default texture pack (just in case) but it doesn't change anything.