But why can't they just use a normal post-alveolar fricative [ʃ]? What's with this Palatalised stuff [ɕ]?
I suppose it was necessary to emphasize the difference between ś/sz ([ɕ]/[ʂ]). Otherwise it would lead to misunderstanding when pronouncing, say, ściera/szczera.
@Timpul: If you use Windows XP go to the Control Panel / Regional And Language Options / Languages (you will have to tick the first checkbutton - to install right-to-left languages; you will also need drivers - either from a backup system partition or a system CD). If you're looking just for an online keyboard, here's a link:
The Hebrew layout there reflects exactly what you'll have after installing Hebrew keyboard on your system (if you decide to). The shortcut for changing languages while typing is left Alt + shift (default, I think).