What is the significance of "seventy times seven"?

Twice in the bible, God makes reference to "seventy times seven" (or "seventy-seven times"). Once in [Genesis 4:24](http://www.biblegateway.com/passage/?search=Genesis+4:24&version=NIV) (dealing with Cain's punishment for his murder of Abel), and the other in [Matthew 18:22](http://www.biblegateway.com/passage/?search=Matthew+18:22&version=NIV) (in an answer to Peter's question of how many times he should forgive a brother or sister that sins against him). Is there any significance to this phrase, and if so, what?