It looks like Greece has some real issues with allowing Macedonia to get invited into NATO. They fell that if Macedonia is accepted into NATO, they'd basically claim the northern Greek region which is also called Macedonia. This has been in dispute for as long as I can remember. When I was visiting a northern Greek island in 1993, there were billboards all over the place that read: "Macedonia was, and always will be, Greece".

Greece has said it cannot support Macedonia's bid to join Nato, because of an unresolved dispute over its Balkan neighbour's name. Greek Foreign Minister Dora Bakoyannis told reporters in Brussels that Athens backed inviting Albania and Croatia but could not consent to asking Macedonia.
The decision threatens to derail Nato's plans to invite all three Balkan states at a summit in Bucharest in April.

Read more here: http://news.bbc.co.uk/2/hi/europe/7280723.stm