Ok...well now those loans are coming due, and Greece has further compounded their problem by essentially lying about the state of their finances. Eurostat (an EU watch dog) recently released a report confirming that Greece has essentially been lying about the bad state of it's finances and revised their budget deficit and debt-to-GDP ratio upwards - http://epp.eurostat.ec.europa.eu/portal/page/portal/publicat...
Then, to add insult to injury Standard & Poors recently downgraded Greece's gov't bond ratings to junk status. That basically means that if they wanted to borrow money from the international capital markets, they would pay much higher interest than say Germany. That was expected though, because news was slowly being released that Greece is in a much worse state than they are letting on. The downgrade just 'solidified' it.
Now, in terms of the domino effect, every country in the Eurozone is connected by a number of elements. The eurozone is the largest economic area of cooperation on the planet (geographically). All the countries use the same currency, but not all are controlled by one central bank. Each country has their own individual central bank. But they all share a promise that no one will let any fail, because that invalidates the purpose of the single currency. It would be the equivalent of California going bankrupt. The Federal government will never let that happen, because it would undermine the entire union.
Anyway, so the other countries within the Eurozone that have a similar fiscal profile as Greece are Portugal, Italy, Ireland & Spain (also known as the 'PIIGS'). By similar fiscal profile I mean that whatever happens to Greece, something similar will have to happen to the others. So if Greece is allowed to bankrupt, then investors fear the same thing will happen to the others. If Greece is bailed out, then the same will happen to the others. It really is damned if you do, damned if you don't, because if Greece is bailed out - then they are essentially being 'rewarded' for their profligate spending and 'devious' fiscal behavior over the years. But if they don't bail out Greece, the entire Euro will likely collapse. So they have no choice, assuming they want to preserve the Euro. If they bail out Greece though, moral hazard is created because the other countries are essentially given a guarantee that the Euro authorities (and IMF) will not allow them to go bankrupt either.
All in all, every countries banks have extended loans to (and bought bonds from) companies, governments, and individuals in other countries. So if the debtor defaults, many banks throughout the EU will have an increasing amount of bad loans on their balance sheet - which could force their governments to bail them out (provided the defaults are big enough). Not only that, but credit will dry further and companies that rely on short-term financing for purchasing inventory and paying employees will go out of business because they can't get access to this financing (i.e. the financial crisis will restart). That will then spread to America, the UK, etc.
I know this has been pretty verbose and might be a bit confusing, but I hope that kinda sheds some light on the situation.
http://www.reuters.com/article/idUSTRE63Q3FF20100427
P.S. Oh yes, did I mention that Greeks don't want the bailout and they don't want to do what needs to be done. They are actually rioting - http://online.wsj.com/article/SB1000142405274870396110457522...
I am glad I don't live in Greece right now.