Yesterday I faced a problem with a PoC I did some months ago. I added a ratings feature to SharePoint 2007 Lists using JQuery.
There were a conflict caused by sp.js and jquery.js using prototype methods. I just followed the recomendations to avoid it.
As I had put everything in my document ready area, I just reassigned JQuery, and use the new variable at that level:
var $j = jQuery;
This should remind us to double check our client code in SharePoint 2007 migration scenarios.