With the text 'abc€' copied to the clipboard, on Linux, where UTF-8 is the default encoding:

Python 3.2.3 (default, Apr 12 2012, 21:55:50) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import tkinter
>>> root = tkinter.Tk()
>>> root.clipboard_get()
'abcâ\x82¬'
>>> 'abc€'.encode('utf-8').decode('latin-1')
'abcâ\x82¬'

I see the same behaviour in 2.7.3 as well (it returns a unicode string u'abc\xe2\x82\xac').

If the clipboard is only accessible at a bytes level, I think clipboard_get should return a bytes object. But I can reliably copy and paste non-ascii characters between programs, so it looks like it's possible to return unicode.