rm -rf problem on ubuntu 9.04

I'd second EmmEff's suggestion. It is fairly common for the system to freeze when some command blocks on I/O operations. This could be due to bad sectors on the disk - check using the fsck or badblocks commands, but make sure you pass in the options to do a non-destructive test. The other, more common, problem is that you're on a network filesystem and experiencing latency in I/O operations.