I remember there was a post a while ago about now allowing external libraries like jquery, but I can't find it anymore. Can someone provide a link? What libraries are allowed?

 I'm not sure if this is what you're referring to or not, but do you mean MySpace's planned implementation of Caja (http://code.google.com/p/google-caja/) to scrub / sanitize user generated javascript?  Caja would have a problem with all of the major libraries at the current time (YUI, JQuery, Prototype, Dojo...) in that they will not Cajole (compile sanitary code using Caja).  Caja will usually blow your code up to about 2-3 times the size and has some trouble with those heavy libraries.  The Google engineers are working towards whitelisting these libraries and will hopefully have that ready if MySpace decides to implement it.  From what I hear, prototype was at a point where it cajoled...whether it worked or not was another story .  

 Anyways...enough with me going on about that, MySpace isn't currently implementing it (see: http://developer.myspace.com/Community/forums/p/746/3128.aspx#3128) so you don't have too much to worry about right now :)...we just implemented a very JavaScript heavy application using numerous YUI components and had no issues. Even if implemented, the OpenSocial JavaScript API will be whitelisted so no worries about that one either.

Here is a slight backgrounder on it: http://blogs.zdnet.com/security/?p=847

I'm sure the nice MySpace engineer's will correct me if I'm going way off on this one :)

When the platform launched, no external javascript was allowed.

A while ago they decided to whitelist certain libraries. I can't find the post listing these.

 Here you go:

http://developer.myspace.com/community/blogs/devteam/archive/2008/08/02/application-guidelines-terms-of-service-updated.aspx

Hey there, I followed up with Chad and he explained that we don't host jQuery but we allow you to use the Google Ajax libs - so whatever google hosts is what is available as a resource for you.

 site:google.com inurl:jquery google that