We have seen Cogent have peering disputes with Level3 and Telia. There are scuffles regularly, usually around the size of the peering point.
With the mergers of MCI+VZ and BST+AT&T+SBC in 2005, conditions were made about maintaining the status quo on all peering connections. However, it was a three year term -- and unlike most telco contracts, this one doesn't auto-renew. Already AT&T is re-negging on its peering inter-connection with, not some small guy, but with Sprint. Read it here.