I am using MS Lync and I was wondering if it is possible to setup an Account in iChat (or Messages in the future) for Lync?
I don't think, that it is possible on client side only. I could imagine, although never tried, that it may work with an Lync-XMPP Gateway. However if you want to use Lync on Mac OS X without the native Microsoft Software, you could try Adium plus the SIPE plugin. It is very limited but should be enough for instant messaging.
Here is an explanation for it: https://www.sit.auckland.ac.nz/Adium_on_OSX