The same problem for HP LaserJet 5100 Series (ethernet connection).
# lpstat -p
printer hp_laserjet_5100 disabled since Mon Jun 21 10:30:53 2010 -
/usr/lib/cups/backend/socket failed
The permisions of /usr/lib/cups/backend/socket were 555
-r-xr-xr-x 2 root root 29988 Apr 9 21:02 socket
After changing permisions of /usr/lib/cups/backend/socket
to 700 it works correctly.
It seems that this is rather a bug of cups, but not of hplip.
Yuri Nefedov.