Have Open DNS Servers been compromised even on a Temp basis or have blocks of IP addresses been impersonated ?
Not sure about compromised, would need to do a little research.
Impersonation of IP blocks is trivial if you can put the right pieces together, but there are ways and means to minimise the impact.
Re the Earlier posting of a bot net been brought down last week ?
One of the biggest use of botnets is launching DDoS attacks, and one very popular form of attack is DNS reflection.
So whilst perhaps not compromised as such, I'm sure OpenDNS see and have seen their fair share of DDoS attacks.