I have installed python 3.0 on Ubuntu 8.10 yesterday and played around
with the new unicode features and had no problems with Japanese
characters(both in interactive and script mode). However, after
rebooting, IDLE will no longer let me input any Japanese characters.
(I have it set to UFT-8, if it makes any difference anymore)

The terminal works fine. It lets me input Japanese and runs commands
correctly.

So I used gedit and saved a simple print("Hello(in Japanese chars)"),
and run it with IDLE 3.0 but I get a blank line >>>. 

(Can I also make sure that I'm opening IDLE correctly? This is how I do it:
1)Open Terminal
2)~$ Python.3.0
3)>>> input idlelib.idle 

Thank You in advance.

I can't seem to reproduce that, maybe it could be a tk issue ?

Can you try writing anything (that doesn't work on IDLE) on a
tkinter.Text widget to see if it shows there ? You could use this code
below:

from tkinter import Text

text = Text()
print(text.tk.call('info', 'patchlevel'))
text.focus_set()
text.pack()
text.mainloop()

I've installed Python 3.1.1 on OSX today.
1) When I use the Japanese input from OSX, IDLE interprets any character 
I attempt to type as a space.
2) When I paste a Japanese string from a different place, it is 
correctly handled. For ex:
>>> print('ここ')
ここ
>>> 

While on Python 2.5's IDLE I had:
>>> print('ここ')
Unsupported characters in input

>>> 

by default.

In any case, IDLE 3.1.1 does not respect the input source and that makes 
it impossible to directly type Japanese (if not other double byte 
character based languages).

Note: IDLE respect OSX dead keys and I can correctly use "Alt+c" to 
input "ç".