We've always been able to use alternative javascript frameworks, but as soon as you do you loose the Liferay tags. As long as you're willing to invest in doing all of the coding in node.js to make your portlet work, you should be fine (there's no real magic in AUI in how it does things, it's just got the most portal tie-ins at this point).
Liferay has decided to fall back to jQuery in Liferay 7, so node.js won't be on the radar.
Remember that what's good for Yahoo is not necessarily good for Liferay. They have two completely different goals in mind. For awhile they were somewhat aligned on using YUI (AUI), but now they're going their own directions...
