Since you already have [QuickSilver](http://qsapp.com/) installed:

    osascript -e 'tell application "Quicksilver" to show large type "your text here"'


You can use this to define a bash function (and put somewhere like `~/.profile`):

    function big {
            osascript -e "tell application \"Quicksilver\" to show large type \"$1\""
    }

Then you can invoke it with arbitrary strings by doing

    $ big "This will cause large text to appear"