The Greek electoral system in brief

In Greek elections, voters select the ballot paper of the political party of their choice, then tick the names of those candidates they support (between 1 and 4, depending on the size of the constituency).
Blank and spoilt ballot papers are not counted.
Parties must earn at least 3% of valid votes to enter parliament. For those who pass the 3% threshold, 250 of the 300 seats are allocated on a strictly proportional basis. The remaining 50 seats are allocated as a "bonus" to the party that obtains the highest number of votes nationwide.
A party or coalition needs 151 seats to secure a majority in parliament and be able to form a government.
To determine the percentage of nationwide votes needed to secure a majority, you should deduct from 100% the percentage of valid votes obtained by parties who did not reach the 3% threshold and multiply the percentage left by 0.404.
Example:
If parties who did not reach the 3% threshold obtained 8.5% of all valid votes nationwide, the minimum percentage of votes needed to secure a majority in parliament is

(100%-8.5%)*0.404 = 36.96%

Update (07/05/2012)

In the 06 May 2012 elections, parties that did not reach the 3% threshold obtained 19.03% of votes. This means that a party with as little as 32.71% of votes would secure a majority in government. 

Following final results, the allocation of seats in parliament stands as follows: 
Nea Dimokratia (conservative): 18.85% of votes, 108 seats
SYRIZA (left-wing): 16.78% of votes, 52 seats

PASOK (socialist): 13.18% of votes, 41 seats
Independent Greeks (populist nationalist right-wing): 10.60% of votes, 33 seats
Communist Party: 8.48% of votes, 26 seats
Golden Dawn (neo-nazi): 6.97% of votes, 21 seats
Democratic Left: 6.11% of votes, 19 seats

Notes
There is no system of proxy voting, early voting or postal voting in Greece, meaning that Greeks living abroad or living away from the place where they are registered as voters have no option but to travel to their electoral district to vote. Travel will be undertaken at the voter's expense, unless political parties choose to subsidize travel for their voters.
It must also be noted that the 50-seat bonus for the first party is designed in a way that puts coalitions of parties (such as SYRIZA) at a disadvantage. The average percentage of votes garnered by coalition members must be higher than the percentage of votes garnered by the first single party (the average being calculated as the total percentage of votes garnered by the coalition divided by the number of parties constituting the coalition).
Example
If a coalition of 5 parties earns 50% of the vote while the single largest party earns 20% of the vote, the average percentage of votes garnered by the coalition will be 10%. The 50-seat bonus will therefore go to the single largest party.