[anghyflawn]

Sure, if you manage to enter the character XeTeX will be happy to typeset it. It's not a LaTeX issue, it's an editor/desktop issue. I don't really know about maths, but I'm guessing keyboard input for many special symbols is hard. I use (Xe)LaTeX to input phonetic symbols, and I hung onto tipa (the LaTeX package that gives you IPA from ASCII) for a long time, until I discovered C-x RET C-\ ipa-x-sampa.

[tikhonj]

You can also use TeX mode so that typing \phi will give you φ :).

Also, the very first time you use an input mode, you can just type C-\ to choose it. After you choose one C-\ toggles between that and the standard one, which is why you need C-x RET C-\ to choose another one instead.

[anghyflawn]

True, but φ is still not the same as $\phi$ :) (and it still doesn't save on typing).

I know about C-\, but I set up my input modes via 'LaTeX-mode-hook anyway :)