Fonts problem (greek) with Microsoft fonts (e.g. Cambria, Calibri etc.)

No problems until Leopard. Enter Snow Leopard.

Now, if I type «Ό» (that is the capital Omikron, accented) in these fonts, all I get on the screen is the non-accented equivalent. Like this:

I type «Ά» and I get «Α»
I type «Έ» and I get «Ε»
I type «Ή» and I get «Η»
I type «Ί» and I get «Ι»
I type «Ό» and I get «Ο»
I type «Ύ» and I get «Υ»
I type «Ώ» and I get «Ω»

I also type «ΐ» and I get «Ϊ». That is, I type lower-case Iota accented with dialetics, and I get capital Iota with dialetics, no accent. Same for «ΰ» (Ypsilon)

If I select the text already entered and switch to, say, Arial, all the errors are corrected; I see the right characters.

Oh, and I've gone into Font Forge and checked the font tables for Arial and, say, Cambria. They have the same characters in the same squares on the grid.

None of this happened until Leopard, like I state above. It effectively means I cannot use the fonts from Microsoft (which, rather surprisingly for me anyway, I happen to like very much).

Any tips? Am I missing something?

Thanks, as always.