These instructions are good only for Fedora 7 and Fedora 8 and are absolutely untested everywhere else. Additionally, they are for gnome; I have no idea what the deal is for kde or other desktops.
Edit your .bash_profile and add these two lines:
GTK_IM_MODULE=xim export GTK_IM_MODULE
If your locale is en_US.UTF-8 you can skip the instructions below and just do (from a terminal window)
su cd /usr/share/X11/locale mv en_US.UTF-8/Compose en_US.UTF-8/Compose.bak cp el_GR.UTF-8/Compose en_US.UTF-8/Compose
Otherwise...
If your locale is not el_GR.UTF-8 (check this by typing "locale" at the command line), you will need to (fro a terminal window)
su cd /usr/share/X11/locale grep lc_CC.UTF-8 compose.dir (where lc_CC is the language and country code in your locale, i.e. everything before the ".UTF-8")
You'll see a line much like
en_US.UTF-8/Compose tt_RU.UTF-8
Note the first value (in the example, en_US.UTF-8/Compose).
mv first-value first-value.bak (for example: mv en_US.UTF-8/Compose en_US.UTF-8/Compose.bak) cp el_GR.UTF-8/Compose first-value (for example: cp el_GR.UTF-8/Compose en_US.UTF-8/Compose)
Install your polytonic keyboard layout:
System->Preferences->Hardware->Keyboard
and choose the Layouts tab. Click the add button and select Greek polytonic.
If you don't have other keyboard layouts installed, go to the next tab which lets you figure out which key combination will change layouts for you.
Now, time to log out and reboot. (When I tried logging out and just restarting X, many apps died.)
This information is modifed from Simos Xenitellis' blog.
Unfortunately the layout has some differences from the monotonic layout (besides just added characters) :-P Even more unfortunately, two of the characters that get overidden are .
οξεία ; + a ά βαρεία ' + a ὰ περισπωμένη [ + a ᾶ υπογεγραμμένη ] + a ᾳ δασεία " + a ἁ ψιλή : + a ἀ διαλυτικά { + a ϋ
These things are unchanged in the polytonic map:
; + . = · (άνω τελεία) R_ALT + q = · R_ALT + Q = · R_ALT + e = € R_ALT + E = € R_ALT + < = « R_ALT + > = »
DejaVu has great coverage of the polytonic code block and so do the Magenta Open fonts. In order to make monospace your default font for textedit areas (and for display of <pre> blocks), for en.wiktionary you should go to firefox preferences (you are using firefox, aren't you?), select Content, clieck the "advanced" button under the fonts section, and set for Western the value monospace intead of courier for your fixed width font, and then magically everything will be better.