I did some additional checking and compared the IT and the NL version of the datepicker file.
The error is present in both languages.
The NL version uses this:
$.datepicker.regional.nl = {
The IT version uses this:
$.datepicker.regional['it'] = {
But that does not seem to be the issue. I checked more language-files and both writings appear randomly. But both times the error appears.
But then again, I'm no JS programmer, I just thought I'd try to gather as much info as possible.

I can reproduce the error at IE8.

This sounds like this problem, just at another language: http://limesurvey.svn.sourceforge.net/viewvc/limesurvey?view=rev&revision=8685

Doesn't seem to be the same.
It seems that perhaps there is a new version of jQuery in the 'main' code of Limesurvey, but the regional settings files are still of an older version?

Opera shows these details in its JS error console:

JavaScript - http://www.rowdyengeesje.nl/limesurvey/admin/admin.php

Uncaught exception: ReferenceError: Undefined variable: jQuery
Error thrown at line 3, column 0 in http://www.rowdyengeesje.nl/limesurvey/scripts/jquery/locale/ui.datepicker-nl.js:
jQuery(function($){

Edit:
Did some more testing.
Verified error is not present in 1.87.
Also, I tested more languages and it seems the bug is present in all languages, so it is perhaps indeed caused by what I described above.

Fixed in revision 8714.

The call for the language-specific date picker was being generated before the call for jQuery or jQueryUI and so failed.

Thanks for the detailed report rowdy.

1.90 beta3 release