"could only be replicated to 0 nodes, instead of 1"

I am trying to install/configure hadoop on a cluster with several computers. I followed exactly the instructions in the hadoop website for configuring multiple slaves, and when I run start-all.sh I get no errors - both datanode and tasktracker are reported to be running (doing ps awux | grep hadoop on the slave nodes returns two java processes). Also, the log files are empty - nothing is printed there. Still, when I try to use bin/hadoop dfs -put,
I get the following error:

# bin/hadoop dfs -put w.txt w.txt
put: java.io.IOException: File /user/scohen/w4.txt could only be replicated to 0 nodes, instead of 1

and a file of size 0 is created on the DFS (bin/hadoop dfs -ls shows it).

I couldn't find much information about this error, but I did manage to see somewhere it might mean that there are no datanodes running. But as I said, start-all does not give any errors. Any ideas what could be problem?

Thanks.

Jerr.