On October 11 between 07:00 and 08:00 UTC, I will upgrade jQuery from
version 1.3.2 to 1.4.2 on all Wikimedia wikis. See  for details,
including a link to a list of backwards-incompatible changes between
the versions. Since we've been using 1.4.2 in trunk for a while now
and most breaking changes were crazy edge cases anyway, I don't expect
much if any JS breakage. User/site scripts and gadgets may or may not
break, but there's probably not many using jQuery (since jQuery only
got enabled on every page about a month ago) and most of those that do
use jQuery won't hit the edge cases that 1.4.2 breaks.
If you are the author of a user/site script or gadget, please do check
it for jQuery 1.4.2 compat just to be sure. If stuff breaks, feel free
to poke me on IRC and I'll help troubleshoot.
Roan Kattouw (Catrope)