[XeTeX] Incorrect capitalisation of greek letters under polyglossia

[Thanos D. Papaïoannou]

Dear all,
Compiling a XeTeX document with polyglossia using TeX Live 2008 turns
	\MakeUppercase{Αιμίλιος ή Περί αγωγής}
to
	ΑΙΜIΛΙΟΣ Η ΠΕΡI ΑΓΩΓHΣ
instead of the correct
	ΑΙΜIΛΙΟΣ Ή ΠΕΡI ΑΓΩΓHΣ

What is the proper way to solve this problem? Alternatively, is there
a hack to temporarily disable polyglossia around the “ή” so that I may
explicitly put in the capitalised form “Ὴ” in the source code? Thanks
in advance.

Yours,
Thanos