We have a number of NetBSD machines plugged into a Cisco terminal
server and they boot fine with unattended consoles. 

The config for those lines looks like:

line 37 63
 session-timeout 20 
 exec-timeout 10800 0
 no exec
 transport input all

A show line 37 shows:

console.mtv>show line 37
   Tty Typ     Tx/Rx    A Modem  Roty AccO AccI   Uses   Noise  Overruns   Int
    37 TTY   9600/9600  -    -      -    -    -      0       0     0/0       -

Line 37, Location: "", Type: ""
Length: 24 lines, Width: 80 columns
Baud rate (TX/RX) is 9600/9600, no parity, 2 stopbits, 8 databits
Capabilities: EXEC Suppressed
Modem state: Ready
Group codes:    0
Special Chars: Escape  Hold  Stop  Start  Disconnect  Activation
                ^^x    none   -     -       none         
Timeouts:      Idle EXEC    Idle Session   Modem Answer  Session   Dispatch
               1w0d       00:20:00                       none     not set
                            Idle Session Disconnect Warning
                              never 
                            Login-sequence User Response
                             00:00:30
                            Autoselect Initial Wait
Modem type is unknown.
Session limit is not set.
Time since activation: never
Editing is enabled.
History is enabled, history size is 10.
DNS resolution in show commands is enabled
Full user help is disabled
Allowed input transports are lat pad v120 lapb-ta telnet rlogin udptn.
Allowed output transports are lat pad v120 lapb-ta telnet rlogin.
No output characters are padded
No special data dispatching characters
console.mtv>exit
On Mar 22,  8:50am, Chris Ross wrote:
} Subject: Re: Boot without an active serial console?
} 
} On Mar 22, 2007, at 8:25 AM, Greg Troxel wrote:
} > Two things to check:
} >
} >   are bootblocks configured to boot with asking?   If you attach to
} >   the console port first, does it boot ok?
} 
}    Yup.  Sorry, I should've mentioned that.  If I connect to the  
} serial port before powering the machine on, it boots up just fine,  
} without waiting for anything.
} 
} >   Presumably the cisco server is DTE and has a null modem cable, or
} >   it's wired as DCE.  Does it assert DTR (or DSR if it's DCE)?  Does
} >   it keep the TXD line in break?   Does the machine boot if you
} >   disconnect the cable?
} 
}    This I should check.  Thanks.  I'll try disconnecting the cable  
} and see if that works, it is possible the cisco is indicating a  
} *lack* of connection.  :-)
} 
}                               - Chris
} 
>-- End of excerpt from Chris Ross