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 [1] 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)