Not seen anything specific just reporters considering all scenarios
If the Greek Government 'cave in' to the EU there could be riots and strikes.If you are well away from Athens or big towns there should not be any problems. Strikes could delay your flight. Again no problem if you are on a package holiday as they will look after you.
Take holiday money in cash (euros) as the banks may limit withdrawals if there is no agreement. They need tourism more than anything now.
The FCO are assessing the situation all the time and unless they issue advice not to travel, holidays will not be able to be cancelled without loss. Even if Greece have to leave the Euro is will take 18/24 month for the drachma to be re-established.