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.
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Please force a crash by pressing F3 + C for 10 seconds while ingame and attach the crash report here.
That's what I got. I tried to force the crash during the freeze, but it didn't work, so I forced it just standing on snow.

Start Minecraft with
java -Xmx1g -Xms512M -cp minecraft.exe net.minecraft.LauncherFrame
pause
and reproduce the issue.
Probably there will be written something in the console window.
I suspect some corrupt soundfiles, probably one of the files .minecraft\resources\sound3\dig\snow*.ogg
You can simply stop Minecraft, delete _.minecraft\resources_ and restart Minecraft.
See MC-2249
Wow! Thank you very much Kumasasa!!
In fact, my file Snow3 and Snow4.ogg were corrupted! I noticed that simply trying to play them on KMPlayer! So all I've done was to delete those files and log in Minecraft!
Thank you very much again! I feared I couldn't play Minecraft any more! XD