On Wednesday evening, I received a mail that a 160 GB SSD was sent and we received it on Thursday morning. Also during the same week, we received a 500 GB 7200 RPM hard drive, which was meant to be a temporary replacement until the 160 GB SSD was available again.
So things are finally solved for good. I am just surprised that the 160 GB SSD suddenly became available so quickly now (it was pretty useless to send a 500 GB disk if the SSD would arrive only a few days later). Is this just coincidence or did the complaining convince HP to finally make a real effort to find a replacement quickly? We will probably never know.