I have a very strange phenomenon on my R-Pi. If I run either
aptitude via a remote SSH shell, the processor jumps to 100% (top shows it's
apt). If I connect my keyboard and run it on an interactive shell (say, by pressing CTRL+ALT+F2) and run
apt in there it works.
When running via SSH I let it run for more than 30 minutes, but it did not continue at all.
Note: I am running Raspbian Wheezy and have not yet tested with Arch/pacman.