It looks like my raspberry randomly powers off.
I thought it would be a power issue, but seeing people rund their pi on 500mA chargers I doubt that.
Sometimes it doesn't have any load and powers off, other times I try to do apt-get upgrade and it powers off after 5 min of installing updates.
The voltage between TP1 and TP2 seems to stay between 4.80 (under load) and 4.90 when idle, and go up to 5.09 after it shuts down.
I am curently running it with only an ethernet cable (no USB/HDMI/..) and a 1A power supply, but also tried it with a 650mA and 900mA PS.
Also it is able to boot up raspbian (2012-10-28-wheezy-raspbian) with HDMI and wireless keyboad/mouse attached on the 650mA PS, but will power off within a couple of minutes.