Currently the collapse/expand state for the centralnotice banner is stored in a domain-specific browser cookie.
For those not using the secure.wikimedia.org interface (which puts all wikis on a single domain), this means they have to click the collapse thingy for every wiki they visit.
Further, if you visit the wikis from multiple computers or multiple browsers, the collapse state doesn't come with you. :(
For people who are logged in with a global account, it ought to be possible to stick the collapse/expand state into some kind of global preference that goes with the account; thus once logged in you'd stick with the option anywhere. This however would be dependent on having an infrastructure for maintaining such preferences, *and* some way to save it. :P
I'm pretty sure we won't get to this in the next couple weeks, but this would be good to target to have done before the 2009-2010 fundraiser.
At a minimum, setting the cookie at one level up might be nice (eg *.wikipedia.org instead of just en.wikipedia.org).