In its latest issue (November 2012), the ADAC ran a short post about reimbursement on delayed flights.
As regulations and global weather change, there are continuous updates on this matter. Here is the latest: If your flight has been delayed by three hours or more, you are entitled to be reimbursed between euro 250 and 600.
This came from the latest ruling of the Europäische Gerichtshof (European Court of Law) in Az. C-581/10 and C-629/10. Their reasoning stated that a three-hour delay equals a flight cancellation.
In case of flight cancellation, reimbursement is generally paid. Therefore, this applies to delayed flights of three hours and more as well.
Please keep the small print in mind – in case of weather chaos, etc., airlines in general do not reimburse. Please check your airline’s website for further details.