Backup from a hot standby

I'm currently testing Pg 9.0.0 alpha 4 and the hot standby feature (with streaming replication) is working great. I tried to take a filesystem backup from a hot standby, but I guess that is not possible since executing "SELECT pg_start_backup('ss')" returns an error? Or can I just tar $PGDATA and copy the xlog files generated during the backup, since the server is in constant recovery mode?  In Oracle it is possible to take backups from a data guard standby, which is very useful since that way making the backups does not stress the master server.

Anyway, I must say that we are very pleased to see HS and SR features, and we hope that 9.0.0 goes to beta (and to an official release) soon!

Regards

Mikko