you're right about this. maybe just because it's readable and so many people already have jquery on their site. and i prefer just disabling the button

  $('input:submit',this).attr('disabled','disabled');

than replacing it with a paragraph, but it's up to you.

You wouldn't use jQuery just to replace a button with some text on form submission. It's a big framework to load for something that small, but if you are already using jQuery then this link shows how to do it in jQuery.

That was my thought too. Why would I import somebody's 3rd party library to save myself typing document.getElementById once on this page?

I'm sure jquery has its advantages for some things. Silly little javascript tricks like this don't seem to me like they qualify.