Boot straight into an application without showing the desktop
If you want to open new windows in full screen by default, I don't know any way to do it in general, but there are options for it in some applications like iTerm 2, VLC, and WriteRoom.

You could also assign a keyboard shortcut to a script like this:

tell application "Terminal"
    reopen -- open a new default window if there are open default windows
    set w to number of windows
end tell
tell application "System Events" to tell process "Terminal"
    if number of windows is w then -- if there are no full screen windows
        perform action "AXPress" of (button 1 where subrole is "AXFullScreenButton") of window 1
    end if
end tell
activate application "Terminal" -- make Terminal frontmost


To automate opening a window in full screen at login, you might use a script like this:

set old to window 1
tell application "Terminal"
    reopen
end tell
tell application "System Events" to tell process "Terminal"
    perform action "AXPress" of (button 1 where subrole is "AXFullScreenButton") of window 1
end tell
delay 1.5
activate old