For the love of god, in all that is holy, can someone tell me what 'menu' key does?

Oh, excuse me vi_. I don't see your question.
I borrowing that from this link: MaemoEmacs .
I using it in emacs.

Made my variant + russian layout with switching by Shift-Space (with combined cursor keys, while english layout has distinct).

At github

What do you think about:
1. Fix Fn-SH-h - dead_tilde - I get "~" only after second press
2. Something for Fn-SH-a, s, d, f, g, l?

Looks like the knowledge I added to that wiki page years ago has been helping people. Well, maybe not, most of the base knowledge was already there, I just clarified some stuff and added the section on multiple layouts (by means other than the stock ctrl+space toggle).

Characters with "dead_" in the front of the name are meant to do that - this is so that you can make letters with accents, by pressing it once, then pressing another character that can combine with it, like so: ñ ã ũ (These also have dead versions: ^, °, ˙, ¨, ", and honestly many others). If you don't need/want to make accented letters, you can change the name from "dead_tilde" to "tilde" in the rx-51 file. Or vice-versa if you want to swap other characters.

About Delete not working in terminal: Use the key "KP_Delete" for the delete key mapping instead of whatever the more intuitive one was called (I already forgot the name of the other Delete key for xkb). That's the one I've been using for years on my N900 and it works perfectly, near as I can tell, everywhere.

...this is the first time I've seen someone post a way to get around shift+fn+[j/k or z/x] printing both letters... You are awesome. Now I might need to rethink my layout, given this new information.

For the record, another way (just reiterating even though it's in the wiki, for those who don't read that part of it), if you find hitting both shift and fn simultaneously a lot more natural, is to simply leave one of those keys at the fourth level completely unmapped. Then it works to just print the one key, for either of the paired keys.

These key mappings are at the X level - they apply to everything that uses the X windowing system or whatever the proper term is - in other words, keyboard remappings and other modifications in the xkb directory apply everywhere on the N900. Every program that takes keyboard input. The only place this doesn't work for that I've seen is the framebuffer console. I don't know of any way to get to console other than at boot on the N900 at the moment though - so this only applies to things like boot-time recovery shells for the time being.

That said, I believe programs COULD override this behavior. Now, to answer your query, with a keymapping that doesn't alter ctrl+[arrow] specifically, MicroB still uses those as tab and shift-tab. Modifying the xkb behavior to allow ctrl+key combos would require you to also modify the file where the groups are defined. Then you can test if Micro-B still overrides... The N900 I'm on with at the moment is about to lose battery power, so I'm going to edit this post from another one as soon as I've re-found where the keygroup behaviors are defined.

Thanks to the useful and detailed info on the wiki; I made my own keyboard layout. It's mostly based on vi_'s keymap but I added on the fourth-level: dead ` on F-key, dead ´ on G-key, dead ¨ on .:-key. Also I re-fixed the Euro-character he removed.

Still, it sucks that qt-application refuse to follow xkbmap, or partially. It doesn't follow all fourth-level keys, however it DOES follow the dead-characters I added on the above mentioned keys... Weird! =/

Edit:
According to Mentalist, some programms use KP_delete. to tackle this once and forever, I was thinking of some construction like this

[CODE
key <DEL> {[ KP_Delete ] };
key <KPDL> {[ Delete ] };
[/CODE]

This doesn't work as expected, unfortunately
Any ideas?

Since a post was made after mine, I decided to add this as a new post instead of editing previous:

The various key level groupings are in /usr/share/X11/xkb/types/

There's many files in there, and they're all apparently read by xkb when looking for the key map 'type'. So if you want to experiment with a Ctrl+[key] key map type, you can just make your own test file in there... I THINK. There's more to how xkb does that then just those files, but it's been years since I've looked around there in-depth.

Well, I'm not sure what you're trying to do, but I have the delete key on Fn+Backspace, and here is the line from my rx-51 file:

I know that just works but from experience with the KP_enter issue (fixed by CSSU) is that certain programms failed to recognize KPEN and/or normal Return. A fix has be implemented to map KP_enter to Return and the other way round, no matter what enter was pressed it will always work. What I wanted to achieve is 100% compatibility

For now I will go with the KP_Delete then