The MacNN forums had a long discussion about Greek in Mac OS X last week. The thread may have been lost when they had their problems last week, though.
Here are the important facts:
The U.S. distribution of OS X 10.1 does not have any Greek support, except that the standard system font (Lucida Grande) supports Greek. Unfortunately, no Greek keyboard driver is included, even though it should be very simple to program, so Lucida Grande is good only for reading. Apple did add a few Cyrillic-alphabet languages to 10.1 that weren't in 10.0.4 (Russian, Bulgarian, and Ukranian), so they are moving beyond Latin alphabet-based languages. (They're hard to find...go to System Preferences, select the International pane, select the Languages tab, then select edit, and a whole list of extra languages appears.)
For now, if you want to browse in Greek in OS X.1, OmniWeb will work most of the time, using Lucida Grande. (The only problem is that OmniWeb does not allow you to set the encoding by hand...if the web page you're reading doesn't tell OmniWeb that it's in Greek, you can't do it yourself, and the page will come out as garbage. Kathimerini looks fine, but To Vima will come out as garbage after the front page.) iCab and MSIE do not handle Unicode conversion properly yet.
Apple originally announced that there would be only 1 distribution of OS X worldwide. However, they appear to be backing away from that. Rumor (unconfirmed) is that Apple is releasing support for several languages in 10.1 but isn't including them in the US 10.1 distribution.
If Apple is going back to localized versions, this is bad news for Greek speakers. The situation concerning Greek (and, according to the MacNN thread, also for Hebrew) in OS 8/9 was unfortunate. Apparently, Apple has/had a contract with Rainbow Computer, the only Mac distributor in Greece, which gave Rainbow the exclusive right to distribute Greek support. This sucks for Mac users in Greece, because they are stuck with a monopoly...they can't buy Macs in other countries and easily add Greek support, and Rainbow can charge high prices. It also sucks for those of us in the diaspora who want/need to write in Greek but can't easily get the tools. If Apple and Rainbow repeat this again with OS X, we're left to our own resources again.
OS X is designed to use Windows fonts, so all of the Windows "long" fonts that have Greek support (including most of Microsoft's standard fonts) should be usable on OS X. However, I tried installing Windows long fonts under 10.0.4, and they didn't encode properly for Greek or Cyrillic. I haven't checked whether they encode properly under 10.1.
If Apple fixes the font encoding, then we can install Greek fonts from Windows, and all we need is a keyboard driver. Some Russians released a keyboard driver for Russian under 10.0.4 before Apple released its official Russian support...we need some Greek levendi who knows how to program to give us a keyboard driver, and we'll be all set.
Keep flaming Apple about this...Windows supports a zillion languages (including Greek) in their standard US distribution; why shouldn't the Mac OS?