osascript -e 'tell application "System Events" to keystroke "LOGIN_NAME"'; \
osascript -e 'tell application "System Events" to keystroke tab'; \
osascript -e 'tell application "System Events" to delay 0.5'; \
osascript -e 'tell application "System Events" to keystroke "PASSWORDHERE"'; \
osascript -e 'tell application "System Events" to delay 0.5'; \
osascript -e 'tell application "System Events" to keystroke return'
Replace LOGIN_NAME and PASSWORD with the proper values, of course...
[robg adds: I broke the above command by adding backslashes to split the lines; it should work when copied and pasted, but ... I haven't tested this one, lacking both Apple Remote Desktop and more than one machine here in the temporary Boston HQ of macosxhints...]

