The root of all evil was a simple function that was searching for certain information in some data and returning me a piece of those, it looked like this:

>>> jQuery.map([{id:1}, {id:2}], function(n){return n.id>1 ? n.id : null})
    .filter(function(n){ return n!=null })
[2]

Looks pretty simple, but it failed in IE. There is some problem with the filter() function in IE, it just barks when I run the following

>>> jQuery.map([1, 2], function(){return null}).filter(function(){})
Object doesn't support this property or method

May be I oversaw some really simple error I made. But anyway, that lead me to debugging this thing and trying stuff out and I discovered that the following happens.

>>> jQuery.map([1,2,3], function(){return null})
[]

But I was very sure that dojo would return an array of three elements with null each. So I tried that of course.

>>> dojo.map([1,2,3], function(){return null})
[null, null, null]

Aha, that’s what I expected. So jQuery just does it a little different. Ok, good to know, this again means that I can cut my initial script shorter, by just removing the filter() call. Good thing.

>>> jQuery.map([{id:1}, {id:2}], function(n){return n.id>1 ? n.id : null})
[2]

So that this works and only returns the id if it is larger than 1, which is actually the case that I need. While I find this quite handy and had wished for it a lot of times in dojo too, I see it as rather inconsistent, simply because it is a special case, you would need to know about.
Since Firefox3 comes with the map() function already implemented (so it must be at least JavaScript 1.6) I tried the native method:

>>> [1,2,3].map(function(){ return null })
[null, null, null]

And as I expected it does return the elements in the array, just like dojo does.
So note jQuery does it differently. Which is kind of ok, since the map() function is implemented in it’s own namespace and does not override the native map() function but I find it confusing.
And dojo just does it as the map() function which is defined in the JavaScript spec.
Just in case someone else stumbles over it …