Seems to be a memory leak at the moment, IMO. I'm on Windows XP, and javaw.exe eats up memory until it's using around 1.2Gb and then Minecraft freezes for me and the only way to release it by ending Java from the task manager.
Ah well, beta is beta. It's annoying but I'll live.
Minecraft is a game in beta. Memory usage is probably one of the things that could use some fine tuning. Java isn't the best platform to use if you want to control memory usage, but you really can't blame the Java Virtual Machine this time. Doing so is ignorant.
Minecraft keeps track of a massive amount of blocks. The memory requirements is probably higher in Minecraft than with most modern "high polygon count" 3d games.
On Windows 7, it can use up to 1.1-1.2 gb of RAM. On Ubuntu, it uses at least 2GB of RAM on the same machine which is my laptop. On Ubuntu, my franerate is around 700-800FPS (Far/Fancy) and on Windows it is around 200-500. I think unused RAM is wasted RAM.